Being immunocompromised means they can more easily get sick when exposed to germs, not that they are a reservoir for a lot of different harmful bacteria and/or viruses. If your friend is sick and within the window for being infectious to others (testing positive for covid, for example) then it is possible for others to catch that particular disease from them. There are rare instances of people being carriers, Typhoid Mary being a case in point, but most people can share colds/flu/etc. only when they are sick. https://www.cdc.gov/respiratory-viruses/risk-factors/weakened-immune-systems.html#:~:text=When%20Sick,a%20respiratory%20virus%20to%20others.