Virus has stopped mom from eating, any advice?

I am uncertain what "virus" has to do with this? The virus, if you are speaking our new Coronavirus, is killed by gastric acids and the virus in not spread in food or by eating. At almost 90 there is to my mind no upside to gastric tubes. They often get pulled out. They cause chronic diarrhea and bedsores which cannot be healed, and which kill. They often cause infection. Those put down naso-gastrically can be displaced, feel into lung and cause pneumonia and death. I would recommend now exploring palliative care and hospice. It sounds as though your Mom is not wishing to live anymore. I am so sorry you are facing this painful time.

During my mom's time in the nursing home I watched a number of people reject food. Some had advanced dementia but some did not, some just weren't hungry and others were angrily adamant in their refusal despite coaxing from staff, in almost all of them it was a signal that they were entering the final weeks of life. Your mom is 89, you need to consider that perhaps this is a signal that she is getting ready to go. I'm going to link a couple of websites for you that I have found helpful in the past:

https://www.youtube.com/watch?v=mNJxq4J5kYY

https://www.choosingwisely.org/patient-resources/feeding-tubes-for-people-with-alzheimers/

My mother lost the ability to swallow following complications of a stroke. She received a g-tube for feeding. It was a quick procedure, and she did fine with feeding by tube for a number of years. I will add that the tube would not have been placed if she was thought to have been terminal, it was done when doctors were giving us great hope of recovery, a recovery that never came.
Even without your visits your mother should reach a point of natural hunger where she’s willing to eat, even if only a small amount. If this isn’t happening I’d guess that her condition may be farther along than realized. Won’t tests be required before placement of a tube? Also tube placement is an outpatient hospital procedure, and I understand those are being delayed right now? What are you being told are the options, other than keeping her hydrated?
I know this is hard, I wish you both peace in the storm