The only thing you will learn is that if your friend could create a $25M company you can do this too. This is how we grow--by imitating our closest peers and we start as babies imitating our parents and siblings. Creating a $25m company is btw not that hard, creating a multibillion company is.
Everything else, stuff you will see and his advices won't help you because once you are in the situation yourself you won't be able to use that stuff. Long story, I'll tell why another time.
Found a company yourself now and let him coach you/make intros to investors and let him check your overall pitch. If you don't have any ideas for a business ask him. This will be your first mission.
Shadowing a CEO without having a clear mission is douchy.