I don't know what happened between them and don't expect I ever will (or should).
I got on well with GF and had invited them both on a trip with me (which I paid for).
They split just before we were due to travel, with the result that neither DS or GF came. I lost the money. At the time I told GF not to worry about it and take care of herself. I think she might have offered something to reimburse me but I cut that off. I genuinely didn't want it, felt very sad for both of them and that money should be the last concern. Considered I'd planned to spend it anyway, so it hadn't cost me anything extra iyswim.
Anyway, they're back together and talking about living together. Yes, I have concerns but not my place to get involved.
What I am concerned about is that "before" she would be at our house several times a week. The last time I spoke to her was when she "let me down" over the trip. I haven't seen or spoken to her since they got back together.
DS says this is just because things haven't worked out for them to spend time at our house. I'm worried it's because she feels uncomfortable, either about the trip/money or the split and some of the things that were said, by her, then, when basically she never wanted to lay eyes on him again.
Obviously I don't want a situation where DS's partner is uncomfortable around me but I don't know what to do without risking making it worse.
If I make a thing of inviting her, that would be very different to the way it was before, when they just came and went and she was always welcome.