This is a warning letter to let a friend know that he or she is in danger of going out with a scumbag.
Friends have to look out for each other, so that's what I'm doing. I noticed that you and <name of rotten person> exchanged phone numbers <description of how numbers were exchanged> and I've got to tell you something. Do not go out with <him/her>. It's not that <he's/she's> a psycho stalker or anything. It's just that <he/she> <description of creepy behavior>. I know several other <guys/girls> who've gone out with <him/her> and they all say the same thing: stay away! I hope you'll take my advice. I know you're a big <girl/boy>, but sometimes you just need a pal to warn you when you're about to step on a land mine. Watch where you step.