On several occasions, his mother, who suffers from incontinence, was left wearing soiled diapers for hours which caused her to develop rashes, Tan said.
Despite numerous feedback, the situation didn’t improve.
Yes, it does need a comma, because it is a nonrestrictive clause adding a little more information about what you’ve mentioned before.
You can easily leave out “which caused her to develop rashes”. The meaning of your sentence will not change.