Burning BM

[The right way]

My friend has an adult son who sits in a wheelchair most of the day and takes a nap in after noon. He has BMs that sometimes burn him even if he is changed within twenty minutes of the BM. They don't happen evey time but if he eats anything with tomatoes or citrus he is affected and it hirts him terribly when he is being cleaned up. The problem is sometimes you don't know what is in the food especially if eating at a restaurant or if it is hidden in store bought foods. So far we think this is the problem, might even be something in his own stomach that causes the reactions. If it is hard he is ok but soft and he burns.
Does anyone know what could help take off the heat so he won't burn so bad? I was thinking about asodophulus but not sure he can take that with his meds. He hasd tried fibre and it might be helping but they still happen.
Hoping for some help here. Thank you.