Hi and welcome to HCM.
Unfortunately, there are situations when chemotherapy
is contraindicated. Every doctor evaluate "cost and benefit" of therapy and you should trust them. R-CHOP is the best chemotherapy protocol for lymphomas, but it is very toxic and has negative affect on the whole organism, so in your case its purpose is doubtful considering your fathers age and general health. Chemo could make him more sick and cause plenty of side effects,and it is questinable would it prolong his life.
I wish you good luck and good health.