“I would never invest in a friend” must be one of the dumbest things I have ever heard.
I get it: if the investment fails, could it erode your friendship? If yes, then your investment analysis and/or friendship probably weren’t very robust.
Investing requires trust. So you can either build that trust with a new person or invest in a friend who you hopefully already trust. Why on earth would you rule out people you already trust?!
Also, whenever you invest in someone, don’t you at least hope to become friends with them as part of working together?
You hereby have my blessing to invest in your friends (assuming, of course, the opportunity meets your investment criteria).