I never have liked the mentality that people have on this site when it comes to being in a relationship and seeing someone else.
I think we all know how easy it is for a woman to end up in the bed of another man ANYTIME she wants. The slut simply goes out to some bar with the intentions of bedding herself a stud and does it. Why, because she has a va-gi-gi.
On the other hand, a man can't simply walk out of his house with the intention that he's coming home with some slut and always do it. Yes, it happens, but it's way more easier for a woman to get hers than it is for a man.
My point in this is, this guy is in a "Relationship". Let's say, hypothetically speaking, they break up today. It's pretty much a given that she will be in bed with another man by this coming weekend.
This is where men and women differ. Ask any woman and she will admit that at one point she was in a relationship that ended and to get over it, she slept with another man almost immediately.
The man on the other hand, regardless if he dumped or was dumped, will try to get in anothers bed, fail, and start missing the ex.
So with all that said, SPIN PLATES, regardless of your relationship status with another. If things happen to end, you'll wish that you had. You get caught, then that's a bullet you'll have to bite my friend.
As far as the girl goes. Don't tell her anything. You don't have to explain anything to her. If she asks who the girl in the pictures are, tell her that she's one of your friends. In the mean time, get more pics of you and other mutual (between you and your girl) female friends on your page. That way she can't single one girl out. It's called DATING. With this plate, you're allowed to be seeing other women, just like she's allowed to be seeing other men. Neither can get upset about the situation because you're not together.