Oh- okay....you are worried about your ability to satisfy her after she's been with him because you're thinking she may actually stretch out so that a smaller penis won't satisfy her any more. I seriously doubt that. you may need to contact Sue Johanson.
I was in a monogomous lesbian relationship for 12 years and we had sex an average of twice a week.....and I had an orgasm every time we had sex.....whereas that does not neccessary happen with intercourse.
So- from my perspective, as a woman, the orgasm itself has little to do with the size of the penis.....or with a penis at all for that matter......my hand held shower massage in the jacuzzi works fine!!
I guess I would ask this.....does she have an orgasm now- every time you have sex..... a real one....not faking it??
If yes- then I doubt that will change.
If no- then you may want to figure out how to change that!
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.