@Jools5 That's really difficult, presumably this person hasn't admitted to themselves that they have some physical limitations.
Unless the stay is almost over, I would tell them you plan to do a linen change, and schedule it at a convenient time, at least this way you can ascertain the state of things, launder what is in there and put in a set of cheaper sheets, maybe a 2nd mattress protector also.
If it's only another day or so, I would say, there isn't much you can do. You don't want to get into the guest's medical issues or embarrass them. Very tough.