I dont know about you, but here's what goes through my mind.
if i were in your situation:
1. i wouldnt have posed as your gf, but thats besides the point. i understand why you did it, but that dosent make it right. Anywho...
2. The fact that you would do it, only tells me one thing. lack of trust in your relationship.
3. The F-A-C-T that he did not have a girlfriend, and she told you he did means that she definately lied to you and that you had every right to be suspicious. Now....the result of all this depends on your threshold.
Me ? i wont be able to trust her...and the question would arise....why would she lie to me about him ?
What should you do ? i think you know the situation in it's entirety and so only you can probably descide what to do now.
But... you aught to realize that you.... have the upper hand.
Hope that helped.
Cheers.