I'm back from the other room, where I was folding laundry....that's when I do important thinking.
With full knowledge that JTT is a pot stirrer, and takes pleasure in taking the opposing side, just to show he/she can, I'm going to address this girlfriend/wife this a bit deeper, to the original poster.
It is always irritating to me when someone, in this case, your former GF, chooses to ignore the fundamental superior status a wife, or husband holds, over all other adult relationships.
Actually, not always just irritating. In some cases, like this one, infuriating.
Your former gf says she feels bad. She should feel bad.....she absolutely did something bad.
The essence of a marriage is that they are the primary relationship, and no one has any place coming between that relationship.
Don't listen to, or try to convince yourself that "oh but, she didn't mean anything by that. she didn't know it would upset his wife, it was just innocently sleeping at this married mans house"
Whether it did actually upset the wife or not is totally irrelevant. The fact she would deliberately put herself in such a compromising situation, where the likelihood of negative consequences would occur, is extremely telling as to what type of person she is.
Anyone who says something like "but you don't know what kind of relationship the married couple has, you don't know if they slept together or not" is being ridiculous.
Put simply, anyone who would think that this type of behaviour wouldn't generally be seen as destructive, is, quite frankly, a moron.
Being a GF or BF is a practice run. Being a wife or husband is the real deal. If she was so tired she couldn't possibly get home, and if for some impossible to comprehend reason the husband didn't make a move to notify the wife (yeah right), why didn't your ex-girlfriend call the wife herself and let her know she was going to be sleeping at her house? It is her house after all, and has the right to know who's sleeping there.
Some people, both men and women, just walk in, create a problem, and can just walk away. They don't have to look behind them at the destruction they sowed. Whether those seeds of destruction sprout or not, is again irrelevant.
Your ex-GF abused you, and she has created dissension in a marriage relationship, and now all she can say is she feels bad? She needs to be saying she feels bad to the people she got between, not you.
And, after all this, you want to buy her sunglasses.
Stay outraged at her.