Hi Vijaya,
If I understand your wife’s issue, she tends to slide/slump out of the chair, which is uncomfortable. Is the air cushion you have one that has air chambers across the complete seat? I had an opportunity to sit on a cushion like that and slid toward the edge. That slouching position is not comfortable.
I have a Permobil F5 wheelchair with a ROHO Hybrid Elite cushion. The front 1/3 of the cushion is rigid foam shaped to your thighs underneath, with firm but softer foam over it. The back 2/3’s of the seat, where your pelvic bones are, are air chambers.
Your sit bones settle into the air chambers, but the rigid foam in front prevents you from sliding forward and slouching. You can get a single chamber cushion or a dual one. (Right and left). I have a dual air chamber model as my core muscles are gone on my right side, and I lean toward the right. I inflate the right side more and the left less to sit more erect.
Permobil owns ROHO, but the cushions can be purchased separately in many sizes. I have included a picture of it from Permobil’s website below.
I have good muscle strength in my legs and arms at this time. To reseat myself fully back in my chair, I tilt myself back so I lay on the backrest, then I push with my feet on the footpads and with my arms on the armrests to complete the move. I believe your Quantum tilts back all the way. It sounds like your wife no longer has that much strength, but you could tilt her back like that, grab her under the arms, and pull her back up. That way, it uses gravity, and it is easier on you.
Hopefully, these suggestions will help.
Len