Mutual benefits

So I feel like this has probably been asked or said before, but what do you do when you find out your fwb had a gf??

Not sure for how long, but while he claimed to be single and never mentioned her? No emotional attachment so it's not a big deal to me but should I be worried about telling the gf? Or confronting him first? Probably a dumb question, but a little on edge. I don't like the sound of being the side chick like that.

Should I just drop him altogether and move on?