It's super important to find out WHY he is doing this. Has he always done it or is it a new behaviour? If it's new, what has changed in his life - is he getting less exercise since your mum hasn't been well, less mental stimulation & play, less treats even if moneys tight from medical bills, is your mum having to go out more often for appointments, is HE suffering medical issues or pain himself being a slightly older dog?
Hope you can find a solution and that your mum feels better soon!