Well, as the mom of a 20, 17 and 14 year old, the first thing that popped into my mind is: what is the matter with him that she wants to protect you from?
Does he have a job, or is he mooching off you?
Does he smell, have poor hygeine?
Is he as well educated as you?
Does he show her respect?
If he meets criteria, then stand up for him. If not, maybe she's got a point.
So, your replies might be: "He really treats me well Mom, the kind of guy you taught me to look for"
"Funny, he only says nice things about you"
"Too bad you feel that way Mom, but I really like him, you'll just have to put up with my choices for now."
"If he leaves bruises, you have my permission to (kill, assasinate, go after) him." (just joking........sort of