Depressed Dad, occasionally lying in bed all day--how concerned should I be?

My 91yo Dad (diagosed with depression and Parkinson's--although that's going OK) lives with my 87yo stepmom (diagnosed with cancer--currently being managed ok). Dad's depression stems primarily from reliving the past (especially a particular bad--but very short and later annulled--first marriage), and some concern about the future. The good news is that he knows he shouldn't be depressed, he can some times talk himself out of it, and he even (he says) does meditation in the morning. That said, he's not taking his meds regularly, and sometimes will lie in bed all day. Stepmom called yesterday, concerned that he was still there at 5.30. The woman who comes in to help (who's pretty darned amazing) says, "Let him be--he's over 90." I talked to him and (I think) managed to get him out of bed. I suggested to Stepmom she needs to find other caregivers in similar situations, given my complete lack of psychological training. Is this whole in-bed-all-day thing a big deal (assuming it's just one day and not, say eight)? Thanks in advance.