There aren't options that necessarily support the knees specifically as they're meant to protect the tendons and coffin bones. But I believe PEI cross country boots offer great support and cooling for the leg. Don't know if this helps. Good luck
Equifits are shock absorbing so maybe that will help to take the pressure off of her knees but I have never seen Boots for knees
They’re aren’t any boots for knees, not to use when riding anyway.