Rental discount request?

[Elouera]

My mother rents out a house to 3 students which is managed by their student association. The student association have forwarded on a request from 1 of the students for a rental reduction, as they say they moved out of the property 2mths ago.

What amount is fair if she does allow a reduction, and would it be un fair on the others? What if the others decide they too will now leave and expect a discount too? Any thoughts or other considerations appreciated.

@wibdib- I asked mum whether the student association were reducing their fees (10%) but apparently not! I guess its worth her asking them if someone else would want to move in (if that's even allowed?), but for only 3mths it seems unlikely. Also, I think the groups that move in are friends, rather than 3 random students being put into 1 house, so they'd be unlikely to be happy about someone they don't know joining them. She doesn't even know if he has cleared his room, or left things in there? Worth asking though.