Just so you know, just because the dog is 120lbs doesn't mean he needs a walk. My German Shepard is about 120lbs and extremely fit. He gets regular vet checks. He just happens to be bred in the Ukraine and a large dog. Do you tell Great Danes to lose weight? Lol. Duke runs in the surf every morning. He's just a big ol' Dog!
Shouldn't be a problem and don't worry, if she lets him stand, she can handle it! One thing I know from owning shepherds is, the females will rule if the male is not dominant enough and won't let him, anyway, unless she wants it! My first GSD (I also have solid blacks) wouldn't let any male near her for 5 years! We had a big male with her, but he was docile and she ruled over him, and never let him get her! Our latest girl, Xena, let the neighborhood Lab get her in her first heat! It was almost like she wanted it!!! Let nature take it's course and you'll be fine!
I would take they 120lb on a couple walks or two then see how he holds up.
Sounds like the 120lb needs some exercise, that's sounds to be a little overweight,if not you just need to watch to see if the female can hold him in the prosess.if not you might trying to help hold her up,sounds odd. But some do it