Problem or clash lies in his chart instead of hers. To resolution in this takes time and his mum acceptance.
Got to gradually work it out with his mum. It won’t be easy. Just being honest here.
Because of the structure, not easy to have a blissful family relationship. You see, there is only 1 water and 3 earth. In each zodiac cycle, there are at least 4 earth and 2 fire to activate this battle in his chart.
Moving out and stay as a couple can avoid 1 aspect of this problem. But the boy parent (especially mum) will still have much influence over him.
You need to evaluate if this is worth the energy. No major issue on the girl chart. Except for husband star is envelope in metal. For she will likely love the spouse dearly and difficult to let go.