Seems like a nice gesture, I see no harm. My mil wrote a nice thank you to my parents for taking lead on wedding planning ( they split thr costs but my mom and i did all the work which saved her lots of $$). We are full grown adults and it was still well received gesture

Anonymous wrote:Don't. It will infantilize your ADULT child. She has a relationship with them. You don't.

This.

If I’d just hosted my 25yo’s boyfriend for a month and he’d been wonderful and thanked me, I’d still very much like at get a simple card from his parents. It’s a kind courtesy and would give me insight into his family. Gratitude is never weird. If they stay together, this is one part of the connection we’d be building between families.

I would thank them being there when she had a medical issue, anything more than that is weird. If she wasn’t staying with them presumably she would securing her own housing.

I don’t see an issue with this. As long as she gave her own thank you (which she did), you’re just adding on appreciation. My ILs live for that stuff and would love it. Just don’t go overboard!

It can be awkward to send a note given that you haven't met, but I think it is the right thing to do. I was in a similar situation but as the host and ended up reaching out to the SO's parents. It would have been nice to hear from them directly after their adult child stayed with me for a while.