I agree. This sounds like a strong case for the "That's nice, dear" approach. You don't have to ingratiate yourself with the other family but, equally, limit your responses to worrying developments to the non-committal. Also make sure she knows she's always welcome to share any worries, but I'm thinking she already knows that; no need to be performative about it.
The picture you're painting of the boyfriend's family is interesting. In my late teens, I had a fantastic Sikh boyfriend. Ticked every box going. His family made me extremely welcome very quickly, and his mum almost made me part of the family by giving me little tasks to do. Having a rather prickly family myself, I absolutely loved this - yet felt a little odd about it.
The lovely Harbanz and some of his cousins kindly explained that, in their culture (somewhere from India, don't know which region), a son's wife moved into his family home and became, basically, an au pair to the mother. His family really liked me or, more to the point, his mother did, and was already making plans to teach me how to do things their way and get assimilated into the culture.
I regretfully ended it; the boys understood. If your DD's in a comparable situation, I'm assuming they're more likely to be Muslim than Sikh because of the drinking & modesty concerns, but the role of mother's servant is the same in households that keep to tradition. It could be worth gently poking around this question if you think it's a likely scenario.
Of course, there are plenty of emotionally dysfunctional mothers of sons in all cultures! It's harder to analyse them without the cultural framework, but DD may eventually realise she's getting enmeshed with the mother and back away.