From what I've seen with my friend who owns a business with their friend, it can be very rewarding, but it does come with challenges.
They understand each other's strengths and weaknesses, share a vision, and mostly communicate well. However, they find themselves not switching off a lot, and talking too much about the business when they get together outside of work. Sometimes they find it harder to make tough decisions too because they don't want to damage their personal relationship. Obviously, if the business fails there is also the financial fallout and how that affects their relationship too, if it were to come to that.
I'd just say it requires good planning, clear communication, and a commitment to maintaining both the business and the friendship, so I wouldn't dismiss it as an idea completely.