I enjoyed the Complete Musician, it's very fresh and approachable, not online course though

https://www.amazon.com/Complete-Musician-Integrated-Approach-Listening/dp/0199347093

https://viva.pressbooks.pub/openmusictheory/

I'm enjoying the Hooktheory books quite a lot.

If your friend is okay with more of a self-studying approach, Open Music Theory covers quite a lot.

https://viva.pressbooks.pub/openmusictheory/

I worked on this free online theory course, your friend might find it helpful: https://soundfly.com/series/theory-for-producers

A lot can be found on YT, but also requires that one stitches it together oneself.

https://www.mmcourses.co.uk/courses is great as curated learning path

Commenting here to find this thread again

Signals Music Studio has an absolutely insane theory course although it's focused on guitar and songwriting

Lots of great YouTube videos. David Bennett and I think a few others have videos ranking music theory concepts but all the videos are essentialy explaining all the basic concepts. From there look up other music theory videos like ones from Adam Neely and Charles Cornell

musictheory.net is a great site for learning the basics.

Regardless of whether you want to go to official classes or not, I'd highly recommend talking to and working with musicians who use the theory (intuitively or with formal education) you're looking to understand. I found that my knowledge went up quite a lot when I started to talking to and working with a lot more musicians. This is not something you need to do formally, just show genuine interest in people's weird music and talk to them.

udemy (paid, but its like $10/8hours of classes) and pdfs

First decide what you need music theory for.

It's an infinite topic, a tool to use with a purpose

I don't really know about courses, but I know of this website that I used for my college courses that covers pretty much every possible subject in music theory that's very in depth that even includes worksheets: https://musictheory.pugetsound.edu/mt21c/MusicTheory.html

Also worth mentioning that you should keep in mind that music theory is not meant to be a set of rules, but rather a form of analysis that explains why things in music work the way they do. Composition is a form of artistic expression, music theory is secondary to everything else.

If you want classical theory, one of the most comprehensive things out there (which is free) is Seth Monahan's video series. He is a phenomenal teacher that is a music theorist and university teacher. He puts all his course material into video form for YouTube. Helped me a TON personally.

Here is the best way I have found to study Music Theory: a free playlist that covers the equivalent of two semesters of music theory at Berklee

Music theory from the ground up (by Ben Levin)

https://youtube.com/playlist?list=PLJTWoPGfHxQH5zdZN6UlMPwZerVApkqmk

I used it like a book, watched it and did all the exercices over a few months and music theory makes sense to me now.

The about/info tab on r/musictheory has a bunch of resources for learning, it'd be a great place to check out

teoria.com is a good place to start!

Learn Music Theory by Thinkspace Education.

Not sure if this is exactly what you're looking for but Alan Belkin on youtube has multiple video series focusing on theory in the context of composition. 

The "Music Theory for the 21st Century" website would be a good review.

r/musictheory

Free options, very basic review: https://www.musictheory.net/

Seconding Music Theory for the 21st Century Classroom: https://musictheory.pugetsound.edu/mt21c/MusicTheory.html

Possibly also Open Music Theory: https://viva.pressbooks.pub/openmusictheory/

More advanced than that I don't know of free resources (but I am watching this in case someone else does!).

Absolutely Understand Guitar is what you're looking for.

Some theory resources to help you :

https://old.reddit.com/r/Guitar/comments/1e8mbeq/what_should_i_do_to_start_learning_music_theory/le89sbi/

https://www.reddit.com/gallery/1fpjeq4

Someone will also recommend this 31-hour playlist :

https://www.youtube.com/@absolutelyunderstandguitar60

As a lifelong student of at 75. I can tell you there's no comprehensive understanding of music theory unless you can hear it--- and once you can hear it, any of these courses have what you need

Having said that, one of the advantages iof memorizing.a lot of music theory stuff even doesn't mean anything to you as that as soon as you can hear it you'll have that knowledge at your fingertips

You can play all the modes in the key of C on a piano easily then you can play all the modes starting on the note C. And you should be able to reference back to all the modes you played in the key of C. Then each particular mode has its own modes and then there are chords derived from all the modes, the modes of modes, and now we are into Rick. Beato book territory

Basically, you can learn all the rules of a foreign language. We're going to say you can speak the language. It doesn't make any sense

they are separated; the one you mention is more classical oriented with heavy use of Musicscore, the other has a title like “Music theory for electronic musicians” or somthing like that and use a DAW to explain the theory

I completed the bulk of it and thought it was pretty good. It's definitely better than getting a bunch of fragmented information off of youtube.

Yes, I did it. It was good; he has a nice demeanor and explain theory very well. He has 2 theory courses, one is more oriented to classical music (The 7 part one) and the other, I don’t remeber how many parts, for electronic music. I think it would be a solid choice for you.

https://www.musictheory.net/lessons isn't a university but they have a decent amount of information available for free

Berklee has great online classes: https://online.berklee.edu

https://thinkspaceeducation.com/

There are some free courses on Coursera.

How about MIT’s Music and Theater classes for free?

You won’t get a certification or diploma, but you can have the knowledge.

https://ocw.mit.edu/courses/music-and-theater-arts/

Edit : Found the Music Theory Courses

https://ocw.mit.edu/courses/find-by-topic/#cat=finearts&subcat=music&spec=musictheory