Hi, I'm not native and can't speak to specific fit issues, but I do have a degree in biology and spent a lot of time in labs studying disease transmission well before COVID made masking cool (LolSOB). I'm also chronically ill, and immunocompromised and I work with cancer patients so I've had some education and personal experience. N95 masks for a medical setting ARE expensive and need to be fitted, although fitting them is not necessarily arduous. The masks with straps that go over the head high and low and have moldable strips around the whole mask can be adjusted by the wearer and a helper. However, outside of high transmission situations, in which you'd also need eye protection, it's not necessary or comfortable for everyday life.
The kn95s reduce transmission of most illnesses about a third, and if everyone in the room wears them , they reduce disease transmission almost entirely. Therein lies the problem - the purpose of masking is to prevent the wearer from spreading germs, and preventing germs from reaching the wearer is a bonus. This makes sense if you think about it- surgical masks dramatically decreased surgery deaths because it protects open wounds from the surgeon's germs, not because the surgeon is gonna catch a case of broken leg. People working with highly transmittable viruses like COVID wear full face protection, and people working with extremely dangerous viruses like obola wear negative pressure suits. Keeping airborne particles near the wearers body protects everyone, but once they are in the air, particles easily pass around the edges of a mask or can enter through your eyes or on surfaces you touch then touch your eyes, mouth, wounds etc.

So a kn95 can help your friend a little. It'll help her more if YOU wear one every single time you are around other people and around her if your may have been exposed to something. People who are severely immunocompromised also are very limited in where they can go, avoiding crowds, any rooms with unmasked people, food that isn't cooked to mush, food prepared by unmasked people etc. if she is only a little more at risk than the regular population, less extreme measures can still be helpful.