بسم الله الرحمن الرحيم
If it is not possible for your father to offer salah sitting on ground in any posture or he feels extreme pain then it is correct for him to offer salah while sitting on chair. There is no need to put stool etc to do sajdah on it, rather just he should bow for sajdah.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.