There are a number of legitimate reasons for refusing to take blood. I offered to give blood for a person who needed many pints. I was turned down because
my blood tested anemic and I had a history of surviving infectious diseases like mono (Epstein Barr Virus)
that would always give me a 'positive' test for antibodies to them. I was told that I could NEVER EVER give blood.

Another reason (which may apply to your friend) is that the blood banks sometimes only need back up on a few blood types. It is expensive to process the blood
and to test it for hepatitis and HIV, so unless they needed his type they might not accept his offer.
Since your friend didn't already know his type, he was possibly wasting their time and supplies.

If he were to go in when they are making a desperate plea for donors of all blood types, they might be far more receptive to pulling out a typing kit for him.

Still, seems like he deserved a more tactful explanation!

I have friends who are 'exiles' from working for a Red Cross blood bank. Appareently, some units aren't trained in diplomacy even with their own employees.
Joan