That is true with the flu shots for sure because they use live viruses. This being an RNA vaccine that can’t give you COVID may be the exception. The only concern is the second dose is a beast, my sub 30 doctor felt awful for a short time after getting it. So your right the concerns with them holding up to that. They have an appointment coming up soon and these questions will be asked.Sometimes people with a compromised immune system shouldn't receive vaccines. I would double check with his/her immunologist.