Re: Back injury and starting aikido?
This particular problem has been asked several times here... but I suppose its pretty common to be afraid when you see falling is 50% of aikido practice.
I started with the same fear because of a back injury sustained as a teen. But I bit the bullet and started training aikido in university. Its an offshoot of ki aikido.
So frankly speaking, it did wonders for my back. (spinal protusion of the lower back). Less pain, and better movement helped. The warm ups I feel we need to learn better ways from pilates or physiotherapists. The ones we did which were general aikido warm ups were probably archaic in nature.
However the best part of ki aikido is the aiki taiso. This helps your body and mind unity. The extending and moving from center and relaxing will be the best thing you do for your body. Strengthens your body without losing flexibility and feeling a complete body movement instead of isolating muscles or parts (which inevitably leads to injury).
Furthermore, as beginners, the more cooperative nature of ki aikido helps gives you a basic foundation that does not stress your body or mind. Forget about arguments on martial efficacy for now. It'll take a longer time for you to achieve that with ki aikido (at least how the others will view it), but on the whole, your return on investment will be higher. (health and mind development).
As your body becomes more intune with itself and skill in ukemi improves, you can explore other perspectives in training.
For me, this would be the best style of training for your friend.