Badly stained mattress

Hi All
My daughter is a student in Edinburgh and is moving into a rented shared house until June 2020, her mattress is very badly stained from the last tenant, she contacted her landlady about maybe replacing it and was told she has no plans to do so, I’d just like to know if she is with in her rights not to replace. As I understand a stained mattress is classed as damaged not wear and tear and advise welcome thanks in advance.