I'm guessing that was a typo, and you meant he has a benign "Lipoma" on his side? A Lipoma is a harmless fatty tumor, and can usually stay in place. But the size of the one you mentioned would normally call for surgery.
Do I personally think surgery to remove this, on a 16 year old dog with serious heart issues -is a good idea? No, I don't. But the pacemaker might allow him to survive the surgery.....however implanting the pacemaker is a risk in itself!
Quite a lot of older dogs go through surgery ok. But it depends on what surgery, and it depends on their overall condition for their age. I they are generally well, the risks are greatly lessened. It sounds as if he is in quite bad shape.
In my opinion, the worst-case scenario would be that he didn't survive the surgery. But if that were the case, he would go peacefully and would feel no pain as he would be under anesthetic.
That, oddly enough, would be preferable to him dying naturally of this heart condition. So my whole viewpoint turns around at that point!
There are three choices as far as I see it:
One -to have him put to sleep and not let him go through any more.
Two -for your mom to continue with the echocardiogram, and see what happens. There is a chance, if he survives the pacemaker implant, that he will feel better! (How long he'll survive beyond that is beside the point. It could be that he will pass away soon....but at least your mom won't have the terrible guilt which often happens after they pass: "I could have tried to help him more. There were things I could have tried, or done, and didn't do."
Three -to let him be, and continue with medication, or to try new medication to relieve his symptoms.
My advice would be in that case -if after a fair trial, the medicine still doesn't work -to let him go peacefully and with dignity.
Even large Lipomas can sometimes be left in place so long as they are not restricting blood flow to organs or limbs, or are too heavy for the dog to bear. Oddly, they can get used to that big soft cushiony lump they carry about. But only you know whether this is causing him discomfort or not. And if the vet has advised it should come off....then maybe that's not a good sign....
The most worrying and important thing is the obvious fluid build-up in his lungs caused by his heart problems.
This is a difficult situation, and I can only advise to a certain extent. I don't really know. It is a hard decision to make.
Ginger did a geat job of analyzing you mom's difficult situation.
I've never had a dog last 16 years, but that may be because we put them to "sleep" rather than go to extreme means to prolong life. We had a Coon hound mix that made it to 15 years and his life was prolonged a few months with steroids, but when he couldn't any longer get to his feet and take care of his toilet needs we gave up. A big lovely and loving Golden Retriever was put to "sleep" at age 12 due to cancerous fat pockets - you mom's dog has apparently had a biopsy to establish his growth is not cancerous. We had the first big fat pocket removed but when they multiplied we decided to put him out.
We have a 9 year old rescue Westie now, he has some health problems, but we think he could make it to 15 or 16, and we'd consider ourselves and him lucky if he see 16 years. These are the thoughts that make it hard to take on a dog (pet) to be part of the family. In our case we are old enough ourselves he could out live us, another difficult and possible situation. Should I see the end of our current dog's life I will not adopt another.