I must admit that I try to be as honest as at all possible, but to focus on what CAN be changed. If the guy has a frail tenor, don't say he needs to drink whisky and listen to Tom Waits, he won't get there anyway.
Sometimes people can't take it - their problem. If you're not friends afterwards, you should help them on building their self confidence, and see someone else for the couple of weeks it takes to let it simmer down. They asked for YOUR opinion, and you can only speak for yourself. Tell them that.
I've learned a lot from those of my friends who are honest with me. I really can't use back patting for anything when I ask for critique/another set of ears.
I'm not saying its easy, but you need to be constructively honest
