Let's see..if I were in your place
If I didn't feel bad after her spending the night with her girlfriend, if she asked me to go spend some nights with her instead of me, and I am feeling fine about it, I might have us schedule 1 or 2 nights a week when she sleeps with her instead of me (while the gf is staying there only I mean, since its not a permanent move in, and hopefully it doesn't turn into that) If after a week or two I was comfortable with that and gf hadn't moved into a new place yet, I'd think about how I felt and be open seeing about a set 2 nights a week. (I would not volunteer this unless I was asked, its important that partners be able to ask for what they want, and it's easy for people to give more out of trying to be nice and then resent it) I wouldn't want to set the precedent to set that she spend more than a couple of nights a week with her, because after she moves out, well I wouldn't be OK with more than 2 date nights a week out with kids at home who need to be cared for.
I'd suggest not to negotiate things post sex or post argument however, and save those talks for early evening or weekends when your mind isn't being colored by being tired or overly relaxed.
I really hope the gf speaks with her husband and clears things up, it seems like you two are being put in an awkward position, and it sounds like something that could cause some problems for everybody.
Happiness will never come to those who fail to appreciate what they already have.