For back problem as people mentioned an upright riding position e-bike is best choice. As such riding position gives little pressure on her spine. Cruisers or dutch style bikes will be perfect.
Check bike to have nice saddle to give extra comfort. And a suspension seatpost will make her ride more comfortable.
Some e-bikes that I can think of are
- Electra townie : Nice Cruiser
- Tern Vektron
- Ariel Rider M-Class