Just sounds like his brother-in-law didn’t get any medical advice, just had his opinion.
Immunocompromised people
The currently FDA-authorized COVID-19 vaccines are not live vaccines and therefore can be safely administered to immunocompromised people, including people with HIV infection or other immunocompromising conditions or people who take immunosuppressive medications or therapies. Although COVID-19 vaccine efficacy is unknown in these groups, immunocompromised people might be at increased risk for severe COVID-19, and the potential benefit of COVID-19 vaccination outweighs the uncertainties. However, data suggest immune response to COVID-19 vaccination might be reduced in some immunocompromised people including, but not limited to, people receiving chemotherapy for cancer, people with hematologic cancers such as chronic lymphocytic leukemia, people receiving stem cells or organ transplants, people receiving hemodialysis, and people using certain medications that might blunt the immune response to vaccination (e.g., mycophenolate, rituximab, azathioprine, anti-CD20 monoclonal antibodies, Bruton tyrosine kinase inhibitors).
People who are immunocompromised should be counseled about the potential for reduced immune responses to COVID-19 vaccines and the need to continue to follow current prevention measures (including wearing a mask, staying 6 feet apart from others they don’t live with, and avoiding crowds and poorly ventilated indoor spaces) to protect themselves against COVID-19 until advised otherwise by their healthcare provider. Close contacts of immunocompromised people should also be encouraged to be vaccinated against COVID-19 to help protect these people.
The safety, efficacy, and benefit of additional doses of COVID-19 vaccines in immunocompromised people continue to be evaluated. Clinical guidance for this population will be updated pending regulatory allowance from FDA. The utility of serologic testing or cellular immune testing to assess immune response to vaccination is unknown. Serologic testing or cellular immune testing outside the context of research studies is not recommended at this time.
https://www.cdc.gov/vaccines/covid-19/clinical-considerations/covid-19-vaccines-us.html