Lending to (or borrowing from) family is about as far as I'd take it, tbh. Unfortunately, friendships can be ruined by things like money; but you're still family even if it goes wrong (generally).
Tbh, I don't think I'd want to borrow from a friend either though. Say you borrow £10k or similar; then you next see your mate out at the pub - would you not be really stressed that he might make some joke about how you should be saving your cash to pay him back, not spunking it on beer?
(could just be me, mind)