I'm at an interesting point in my life right now - I've left my full time job to focus on my own business. As I left, he offered me this opportunity.
So, what have I got to lose? Time. Time to develop my own business the way I want. I have much more faith in myself than my friend and his business.
But as you say, 30% is very good for an established business which is why I'm stuck!
The good thing about business is that there is NO Right or Wrong way to develop a business. If you already have a MVP in place already, I think you should work on your own business, bring it to the next level.
If not, then maybe you want to take up this opportunity - to learn & gain experience, build your MVP, save up, etc.