I have had this, especially if your friend is somewhat passive.
I went through a period of caring (because it was knocking my friends confidence) and here is what i found out. Its a shit test!
If you ask, why? or what don't you like about him, or say something like, "no, honestly he's a cool guy, i promise") You lose!
So answers i use.
"i don't like your friend"
I didn't like you when we first met, but now iv'e got to know you, i can see, deep, deep down, your a little shit
I bet you don't like most people you see, but oce you give people a chance to surprise you, they can be really cool.
Yeah, he kill's kids. (ok i have a wierd sence of humour and can pull this off, if you cant try)
Yeah, he sells drugs.
These next two work on the asumption your friend has alot of balls)
Hey, (friend) this girl says she doesnt like you? whats up with that? (he should give a cocky funny comment and join in talking)
Who that guy? i though he was with you. (If you ever what to fuck your friend over for no other reason than its funny....)
hope these help