I get a proximity effect when I wear headphones that changes the pitch of what I hear, but I am still capable of intonating to match what I hear. When I record and play it back through speakers, everything sounds a little more flat than it did when I had the headphones on, but nothing sounds out of tune to the music itself.It's common for the perception of pitch to change as people get older, sometimes by more than 2 or 3 half steps. This would affect both singing and improvisational instrument playing because it affects the ability to accurately match your performance to reference notes.
Because it's neurological in nature the person often isn't aware of the difference and there really isn't any practical solution to the problem.
It's not possible to determine if this is the cause of your bandmates problem without a legitimate diagnosis by a medical professional.
Edit: Some medications can cause changes in pitch perception. Carbamazepine is one of these. It's a neurological medication used to control epilepsy.
I would think that someone with a problem like you describe would still be able to hear that their voice is no longer pitch matching the instruments when they hear a recording.