Never an easy choice. Both our girls have moved back home in the past.
A couple of things you have going for you this time 1) as someone said your DD is a mother now so has her son to think of too. 2) her present home is less that idea and she wishes to improve on that.
So a reality check (if needed) that you and OH are a very important part of helping here her get to the better life she's hoping for. Or she can go back to what she is wanting to move on from. Facts that you may be didn't have when she was at home before.
I also agree with the folks who said you may regret not helping.
I would have regretted not helping our girls.
No 2 DD was an awful teenager, she came home from a relationship break up. Having her at home went well and was easy apart from the obvious emotional fall out and support.
No1 DD, a very easy teenager, aged 40 at the time of return, SIL and DGD aged 13 months returned to UK from New Zealand after the Christchurch earth quakes with luggage only.
Not such an easy time. Intact surprisingly difficult. Especially as over the years I had spent months in NZ at their home without any difficulties. I was with them for 3 months for the birth of DGD and had gone through the big Christchurch quakes with them so thought it would be easy.
They had PTSD post quakes, very firm views on child rearing that sometimes came across as we knew nothing about bring up children. Plus, what was left of their home was in a container somewhere at sea so moving on was difficult.
Apart from when I was working this was one of the most stressful times of my life.
However, after 8 months they got their life back. They and OH can now see what would have made the time together easier but we all survived and had some lovely times too.
Who knows how it will really go. Good Luck and I hope for you all that you find a successful and peaceful way forward.
Our Welfare State. Is it broken?