While I know hearing what you just heard hurts like hell....AT LEAST HE WAS FINALLY HONEST WITH YOU, at least to a point...you may not have received the full and complete truth, but you did at least get your suspicions confirmed. At this point, knowing what you know now...do you still need to see that bill or can you move foward basing decisions on current actions and current verifications? For me, the fact that he FINALLY came clean is important.
You two are going to be seeing that ST soon, his inability to go NC with this woman needs to be brought up.