I am trying to help a (male) friend who suffered back injury in a serious automobile accident several years ago. He cannot walk or stand for more than a few minutes at a time. He is a former US Army servicemember and is used to being much more active than his injuries allow. He has managed to keep in excellent shape, but has been unable to find any way to work his abs. Are there such exercises or any equipment that would facilitate this? He is 48 years of age.
The abs and the back are two sides of the same coin. Anything you do to one will affect the other. And any imbalances between them will be a potential for injury. In short: you cannot work your abs without working your back. All the exercises that strengthen abs also strengthen the back, and vice-versa.
Unfortunately I don't know enough about your friend's condition. I'm assuming by the context of the question that the reason your friend can't stand or walk for more than a few minutes time is because of pain. Since you mentioned the back injury was permanent, my guess is that he suffered some form of spinal disk injury or worse fused some parts of the spine together.
The function of the core is to provide stability to the spine. In many regards the best, and possibly only, choice your friend has is to work his core isometrically. In other words, his spine should not be in any sort of flexion during the exercise, but he would be tightening the abs as he works with the resistance. The family of exercises that would allow him to do that are:
The thing that each of these has in common is that the core remains stationary, and you can increase the load as you can handle it. If you can find more core exercises that share the same features, all the better.
I think the most important bit of advice for your friend is this: it will probably hurt, so start with nothing on the bar and see how that does you.
I would probably start with landmine twists since you can do them seated. You will have more leverage from a seated position, so the load of an empty bar from this position might be bearable. I would suggest starting with the bar in the top position and slowly lowering it left and then right as far as the pain will allow. Try increasing the range of motion each time until he can do all the way down.