You're more than welcome and I hope my response will reassure you about your friend's cat.
Without direct visualization or possibly a picture, I can't say for sure of course, but it sounds as if what you're calling a blister is a normal part of this (or any) cat's mouth, especially since what you're seeing are bilateral (on both sides) and if they are located on the lower gums.
I've included two pictures below; see if these are similar to what you're seeing. In the one picture, the swelling is a darker pigment (which is normal) while the other one is the same color as the surrounding gum color.
Cats can develop oral masses but it would be quite unusual for them to develop in the same location of both sides of the gums.
As to the darkish discharge around his nostrils, this may also be normal for him.
A small amount of moisture can collect around the nostrils (more so in some cats than others) and dust particles and dirt can collect there. Many cats will rub off this material during their normal grooming but some cats are a little lazy about this and the black material builds up.
I tend to take a warm washcloth and gently clean the area when this happens to the cats that I've had over the years.
I hope this helps and provides reassurance about these issues with your friend's cat. Deb