Try soaking mattress with white vinager and leaving in sun to dry? I agree with the poster who said get a plastic sheet cover to protect the bed.
Maybe put a litter box in the bedroom for her to use rather than the bed? Make sure it is always clean for her?
Petco/Petsmart sell natural pet deterrants made from things like bitter apple that may keep her off the bed and won’t smell.
:( I also have a problem with cat urine. I have never dealt with this before so I’m looking for some helpful tips. I took in a stray cat this winter and she has been urinating in her litter box as well as on the blankets/bedding and the mattress and carpets in my guest room. I was told she was too old and weak to get spayed b/c I thought maybe it had to do with her being in heat. I have noticed that when the litterbox isn’t clean enough she will use the bed instead or if she is aggrivated by my other cat she’ll use her blankets etc. I’m looking for a way to remove the odor mostly from the mattress. I have been able to remove the smell from the bedding and carpets using something called trizyme which is a detergent booster. With the mattress though it soaks in. I started with a solution of the trizyme and covered the mattress where it was soiled with the solution, let it sit and then used my carpet cleaner to suck the water up. It still smells and I was wondering what my best bet would be as far as house hold remedys and how to deterr her from using the bed.