"My Friend's Starting a Business. Wants Me to Join Bro!"

Say yes if you want to waste your time and money and lose a friend in the process.
Business partnerships work only in a very narrow range. The textbook case of a partnership that works is a law firm or a medical practice. The partners are all lawyers or doctors, so they're not complementing each other but expanding the firm's ability to serve clients, and their duties are very similar to one another. Hence it really is a partnership and there is no unnecessary conflict. The paralegals or nurses, they're always employees, not partners.
Unfortunately, people took this to mean every business should be a partnership, which is the furthest thing from the truth. You simply can't have partnerships where the partners are fulfilling different roles in the business. Whoever is the only person who can do the thing the business sells, that's who has to be in charge, and everyone else a hireling.