I've received a couple skirt comments from this one guy at work, but what your coworker did crossed the line IMO, especially because of the presence and same reaction from his girlfriend (it sounds like harrassment, whereas my co-worker's comments sounded like good natured ribbing)
If this is a first time offense from him, I guess I would probably ignore it for now and wait to see if he keeps doing it. However, if he's made comments about it before and this is an ongoing thing, then the matter needs to be dealt with. I would definitely try to fix it (either by reacting to his antics or talking it out with him) without going to HR/management first, but if he doesn't stop, then I'm afraid that's where the path is heading.