Heart murmurs can vary greatly in terms of pathological effect. They’re primarily congenital.
My dearly departed Velvet was diagnosed with a heart murmur shortly after I got her from the shelter at around 8 months old when I brought her in to be spayed.
I was initially very surprised and concerned and asked them if that meant an operation or what else could be done.
Every vet I ever took her to said there wasn’t really any treatment for it and that I should just keep an eye on her when doing anything strenuous like chasing around a lot or running up and down stairs, etc. for any shortness of breath. (as if you can prevent a cat from over exertion :) ha ha
She outlived two of my other cats, both younger than she, and lived to the ripe old age of 19 years old. And even then, it wasn’t the heart that did her in.
Fortunately, she was definitely the type of cat (like Garfield) who spared herself any excess exertion anyhow, so I never had to worry from that standpoint :)
So you can reassure your friend that every heart murmur is not automatically a quick death sentence. It sounds much worse than it is many times. At least that’s what I was told by several different vets over the years.