Depends on how you say it and when you say it. I've had my boyfriend's friends say that to me at times, when they really just meant they're happy I was with him because I treated him right.
If you don't really know her boyfriend and just her, it could imply you were interested in her.
She'd probably just smile, say thinks. The way anyone would respond to a random compliment.
Unless you're like her ex or something. Then it's just awkward.
or unless you say it in some dickish/creepy way.