Very good morning doctors. why might my very good friend who is also age 61, who is fully vaccinated for covid, get a blood test, and then test negative for antibodies? could this negative antibody test be the antibodies may not be working/are absent?

Protective antibody levels are adequate after two vaccination dose and two weeks after the last dose. The timing of the antibody testing should be identified. You may re post the question with this information. If the response was not adequate, one possible reason is an immune defect.

Which test ? The antibodies to the various vaccines differ from those of the wild covid disease. Vaccine antibodies the spike protein not the main virus, so those must be tested differently.

There is not need for the test and there is no prescribed subsequent action based on the result you may get. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health!