The last support group meeeting I went to I met man who had back problems...I asked him what he does for exercise...Or what can someone do...We know things like swimming or walking (if it doesn't hurt) will be good. He mentioned this below...It is at many physical therapist offices...A recumbent cross trainer.
Just throwing it out. We get this question almost weekly on the Exercise and Fitness Forum. Big bucks...But just an idea...

PhysioStep RXT-1000 Recumbent Elliptical Cross Trainer $2,450 online Recumbent - With Heart Monitor HCI PhysioStep RXT-1000 Recumbent Elliptical Cross Trainer - Light CommercialOne of the most innovative pieces of cardio equipment on the markettoday that you should consider for your gym is the HCI PhysioStep RXT-1000Recumbent Elliptical Cross Trainer. Thismachine takes the elliptical machine but puts a recumbent bike spin on it asyou'll be seated while performing the motion. In addition to this, the machine will also offer a great core workout toyour users, which is something that many gym members will be seeking

 My normie husband is a back pain guy and uses our Tread Climber with the approval of his back doc.  It has been worth every penny, for both of us.