Advice requested. I don’t know the best way to answer her or how to cope with this.

She needs to know.  Tell her that at the time, it wasn't a good time to tell her, but now things have changed, so now you are telling her.  She is asking, she has a right to know, and she probably suspects something anyway.  They were a couple at one time, and the longer people live together, the more in sync they become, so she knows.  Just verify it with her.

My Mother is 88 yrs old and very forgetful. There would be time she asks where is my father and I’ll be truthful about it that he passed away several years ago. She accepted my answer.

Hurtbabygirl, I;m  so so sorry to hear of you Dads recent passing heartfelt condolences to you in your grief and in the ongoing responsibility of you mothers serious  illness.. It is a hard and emotional time for you Take care, I would say work with your own natural compassion  for your mother, but listen to hospice view too.. We had a similar situation where my mother had mild  fluctuating to moderate dementia  being cared for in her family  home for 4 yrs with my father and family. My father who was in good health was in hospital having medical investigations and suddenly was on hospice care with medical treatment withdrawn without  he or family  being informed of situation- very poor medical team.  All very upsetting and confusing for him and family. We managed to get him home for his final 6  days with hospice care which was his wish. He was aware but barely responsive and my mother was in disbelief, we had no preparation to how to handle it , we  told her that  Dad was seriously ill & unlikely to get better but did nt quite say he was dying. She thought he would get better in time now that he was at home but she spent lovely time with him, holding his hand and talking to the hospice nurse about dancing and what a gentleman he was. Saying at other  times he was nt her beloved but was another nice man with the same name, when he looked very sick, but she would help this poor man. It was a coping way and we let herwork it out with hospice overview. She was so peaceful, stroking his cheek and relieved that he was out of the horrible ward where she had visted him . They were together for those 6 days on and off and much of their communication was non verbal. My Dad knew she was there. When he passed, hospice nurse said to tell my mother but she might not take in the death but if  if it  became a repitive distressing  question with memory loss  to ask Dr to prescribe a small dose of a med called Serenase which helps the mind get out of a loop asking same question if forgotten . Family did not want her to go any part of  the  funeral but home team advised some part would help her acknowledge   the death., lot of family had differing opinions so in the end  she did nt take part, But she also grieved for being left out ;as it made it  unreal, also wondering what kind of wife she was  for not attending ,   We so wonderfully  had my mother with her dementia and copd for a further 7 months in home care,  as we were so let down by hospital no on wanted her to go there if at all possible. Initially she questioned about  Dads death  intellectually as the sudden death did nt sound right as (she was right about that) & felt I had not told her the truth and could nt believe she did not  go to her husbands funeral & ,was a bad wife. We talked a lot and very honestly  and that helped her. She had a few good months out and about on her walker visting  and having coffees questioning it all in her mind but not too emotional.  Then the  impact hit her and she cried  deeply and  inconsolably  feeling the pain of her loss. Her sister told me she was never able to cry with grief before even with her parents & sibling passing . They said  it must be a big relief for her  to be able to cry as so  often she could nt. She knew Dad  was now  gone t  " her  great  protector " through their yrs of marriage and there was no confusion. She developed pneumonia once the grief and inconsolable grief started to flow. It was heart breaking but she was able to process and release .for someone who kept all in before she died. It was painful and we were blessed to have the most empathic and kind  home carers calling who brought in great love and were able to support her and us  with compassionate kindness and lot s of hugs and warmth to help her through her grief sadness, guilt , despair  and reached a sense of peace before she passed 7 months later at home with us.. She got the chance to grief the husband she loved so much and was ready to leave herself then.

Hurtbaby girl   Sorry my Post went a bit long but may be something there.will help you. Its hard to  make decisions when grieving your Dad. There is  no way of knowing  what is absolutely right for your mother, but try get whatever support you can in the  facility for yourself as its a responsible decision on your shoulder in the middle of your current grief.  Maybe  Dr can make a plan to manage any adverse outcomes if it does nt go well. 
Does she have a geriatrician who looks after her meds who might advice in aftercare re meds if it affects her wellbeing emotionally or psychologically
Hugs and try a get yourself some sleeps as well, helps  everything. xx