Upgrade her
I fly over 100 segments a year (most long haul) and all in F. So if I end up in F and anyone else is in C or Y I make sure they sit upfront.
If I fly with family of GF I always either
1) buy C/F awards
2) buy C tickets
2) upgrade to C with miles or
3) in one case where I was in F and others in C gave the F seat to the female in the group.
Actually there is one case where a NR passenger (mother of a pilot - about 70 years old, was strapped into the FA seat at 1L. I saw it and got up and told the FA to swap with me -- I think it was wrong to have the old lady sit like that -- she had no clue what was going on.
Well in the end I was asked to sit in the jump-seat in the cockpit. (Yes I have the credentials for such a treatment).
This has nothing to do with gender BS, but I think it is normal polite behavior.
Yes I travel a lot, but it is nice for others to experience a little bit of luxury as well -- and I can suffer a little bit.
and if he needs to know how to do it...
At boarding go to the purser, show both BP, and tell them you are switching seats so she can stretch out, and the he is in seat XXX.
The FA will love him for that, he will get nice treatment from them, and if he is super lucky he gets the seat next to her.