Wait how?

Because he isn't working out "for her" or because he doesn't think his grandma is strong enough to throw her own car batteries in the ocean?

If A) I'm assured women aren't dressing in a certain way or doing makeup "for men" as well, they say it's "for them." Unless this is misandrist, I don't think working out "not for women's approval" is misogynist. Doing things for other reasons than to garner the sexual attraction of the opposite sex seems like an acceptible path.

If B) It could just as easily be that he isn't doubtful of grandma's abilities because she is a woman, but because she is old. Maybe it's ageism at play instead, or maybe she asked previously if her grandson could assist with routine car maintenance such as a battery change.

If C) Other, please inform, as I have missed it.