Should I tell this guy that my friend was flirting with to back off
Why do you want him to back off?
A friend of mine was flirting with him, but I don't think he's really interesting to her. He seemed like an okay guy to me, but I don't think he's her type.
It seems like you are getting involved where you are not needed. You should let them figure it out on their own.
I'm going to figure it out without you.
Are you sure she's isn't interested in him, since she's flirting with him? You could ruin a situation that your friend is enjoying by trying to intervene.