Entering the plane is actually more difficult. Egress is pretty easy, especially in a low wing. To get in the best way (make sure the wing is cool &/or bring a blanket)is for them to just pull up to the back of the wing and flip up. At that point it depends on upper body strength. For strong people I'll have pre rigged some lines with loops on portable hooks and I'll also rig an overhead grab in the middle. Since you say she has little upper body strength, it's really a lot easier (as long as you can handle their weight) if they just give self reliance and independence over for a couple of minutes and let me hand toss you in. I've gone through this issue before with handicapped clients and guests. I just approach them and make it plain that there may come a time when someone may have to throw them over their shoulder and run and they are all good with that. I then go further that due to the general difficulties of getting around a boat, that should they prefer that someone take them, that someone will always be available to do so. "If it's rough, please take help" (whenever I know that we'll have wheelchairs onboard, I always put on a couple of big guys).
If they are good with being handled, it's pretty easy, otherwise have a 5' 2x4 ready to serve as a lifting lever.