It was an exceedingly confusing OP.
OK, to try and unpick this.
I am a woman and if my male partner and our straight female mutual friend went out and she stayed at his and was wandering around his flat in her knickers and t-shirt. Then posted on fb saying 'happy the morning after' I may or may not have raised a suspicious eyebrow. It would depend on the dynamic of the relationship between the three of us.
Most women do not loll about in their pants and tee in front of straight male friends, especially straight male friends who have partners. Some do, entirely innocently, I'm sure.
However (and I can't claim to be an expert on the dynamics of lesbian friendships and relationships) whether you are straight or gay you have probably grown up spending some time in the company of partially dressed women. In school changing rooms, on the beach, in friend's homes. So even though in the case of lesbians other women are the potential sexual partners it's 'normal' for them to be in each other's company partially dressed.
So, you know your gf and friend better than we do, but in itself I think this is not a massive sign that something has happened between them, or that this is inappropriate behaviour. I would think happy the morning after is probably more a reference to a good night out and possibly still being a bit drunk or even just not hungover.