I think it's much harder to remain motivated on your own project on a completely independent basis. This seems like fun, in that he's "being paid" (by himself, admittedly) for the 25 hours and might continue work at his normal rates, so it's extrinsically motivating, but still his choice of work, instead of being whatever-he-can-get-hired-to-do, which is the usual freelancer dog's breakfast.

I have several side projects which I do work on, but working on 25 hours for free on someone else's project is more likely to lead to future paid work than working for 25 hours on my own projects.

I also find it hard to justify working on my own projects when there's client work to be done.