Go to Targer, K-Mart, Walmart and buy the large pads for her to lay on. My mom is invalid and lays on these all the time and is used to "collect stuff". I suggest getting the largest size. Lay it on top of the bottom sheet.
These will not cause any skin irritatioin. Mom has laid on them for 4 years and never a rash.
Poise is one brand but we've been using the store brand...much cheaper. In the incontinence area. PM me if you want.