Re: Back injury and starting aikido?
I have seen this movie before! I had a traumatic back injury years ago in the Service. I had always been very physical, and into martial arts of any type. I train regularly and would do more if I could (family obligations). I am not limited by my back injury, but I must always consider it.
This is a fact - Everyone is different. Your partners back injury is his, and all the advice you get here has to be filtered through that fact.
With me, I know the more active I stay, the less problems I have with my back. I can break-fall, and train hard - but that is me. In fact I think rolling is one of the best things I can do for my back.
I would say - observe a class, go slow, and monitor the situation.
As for Doctors - I have had a few! Read the research. Many are saying that surgery is often unnecessary. I would focus of PT and find a good Chiropractor. Any professional I would see would have to be "sports" oriented. A doctor will tell you that falling, rolling and extreme physical activity is risky.
Two short stories: A friends wife took Aikido and was diagnosed with scoliosis - the doc said quit - she did. I have not see either of them in years. Another had a broken back. Doc said "no way." He slowed down, paced himself and is a shodan in Aikido now. Both true stories - like I said, "everyone is different."
My $.02 -