Mom is facing the loss of her pet companion. Any suggestions or advice?

The best thing to get over a loss pet is another pet. With the elderly it might be better to be a foster than get another pet that can live 20 years.

Yes we all handle these things differently don't we? There is no "one size fits all" solution to end of life issues for our beloved pets. I had a precious dog companion. After 15 golden years his kidneys began to fail. There was no way I would allow the faithful old boy to suffer, and no way I would let him go into his last sleep alone. Giving him a peaceful, painless end was the last, best gift I could give him. He fell peacefully asleep wrapped in my arms. I will always treasure that moment. No fear. Just trust.

My mother went through this with the loss of her little dog. We did not know that she cried herself to sleep every night. As soon as we did my sister found mom another little dog and he is the magic pill to mom’s contentment. (One of us will take the dog when mom passes.) Go to the pound and tell them you want a cat who wants to lay in a lap and be petted all day long is my advice.

I'm going to assume the cat is much older than 5 yrs (and she's just had him 5 yrs) or has some unusual disease, rather than that the cat is only 5, which may mean the prognosis is less grim than it seems.
It's hard for ANYone to deal with the loss of a pet. They are the only ones in our lives who don't judge us negatively based on our looks, status, job, or something we said wrong.
Depending on your mom's health, I suggest that you search online at Petfinder for SENIOR pets needing a home within your zip code. It will pull up only rescue groups who have cats or dogs who fit the criteria you input. Because they're rescue groups, not individuals or county shelters, they will charge an adoption fee that covers vet care that would cost you a LOT more if you went to a vet yourself: spay/neuter, shots, deworm, etc... It will take a few days to pick out the right one and go through their application process, so, by that time, your Mom may be past her worst grieving and ready to accept another in her life.
There are a heartbreaking number of senior dogs and cats out there (and middle-aged ones) because many people decide they no longer want the responsibility, so give them up or move and leave them behind. Most people want a puppy or kitten, so the adults get overlooked.
You can check with the local Humane society and city or county shelters as well. They're also likely to have seniors and are more likely to be in the unenviable position of getting so many daily that they have to euthanize a large percentage.
The downside is that don't usually know as much about the animals as a foster home based rescue group.

They sell lifelike therapy plush pets . One brand is Joy For All. I’m planning on getting one for my mother once she has to move to memory care since they don’t allow real pets. I’m sure there are other brands, they are roughly $100