I am half Chinese and grew up in Asia. I doubt that the mum is an asylum seeker or intends to skim off the NHS or whatever nuts conspiracy theories on this thread, if this student and her mum are from China. It’s very common for Asian parents to be hyper involved and if this girl has never had any experience of caring for herself then her mum might stay with her.
The question is, is this just for a few weeks to help her settle in and find her feet (agree it’s extreme and inappropriate by western standards but they may not be aware of this) or is it intended to be permanent?
As others have said, your DD and her flatmates need to talk to this girl and ask her how long her mum is staying. Say they understand if she needs help settling in but it’s against the rules for her mum to be staying in halls with her long term. Say it politely, not as a threat, but make it clear that it’s been noted and the flat mates aren’t ok with it.
If mum wants to stay longer to help her DD settle into uni, she needs to find and pay for her own accommodation. Her immigration status is no one’s concern imo, but she needs to get out of halls. Again, it may be a cultural mismatch where mum and daughter think it’s totally fine but the other young people in halls who have gone to uni to bond with other young people and experience independence alongside getting an education are being shortchanged. This girl is also being shortchanged of finding her own way and making friendships. Many thousands of international students manage fine without sharing a bed with their mum every night.
The above assumes the mum and daughter are Chinese, and obviously being Chinese is not a monolith and I can’t vouch for or explain the behaviours of over a billion people. But I think the cultural background and therefore potential for misunderstanding is relevant.