If an LOR from a job is okay to have, I'm pretty sure that one from a retired professor is more than fine. Besides, how would medical schools know that he is retired? (from my understanding the probability of adcoms contacting him/her is slim to none, but I understand your concerns nonetheless)
(One last thing: I am NOT against LOR's from jobs, I was just using that as an example. Don't start hating me SDN folks.)