The verb gelten uses für to designate to whom it applies.

Also, I think it should be ‘irgendwas mit den Fällen/Kasus zu tun?’ (assuming you meant to say ‘something to do with the cases?’), but I don't know whether you can use the expression this way.

Yes, it means “to be valid, to hold true, to count” or “to be of value”. The first meaning is being used here: “physics holds true for her too”. Another example could be “diese Regel gilt immer noch” (“this rule is still valid/still in force”) or “das gilt als vollständig” (“that counts as/is deemed complete”).

gilt auch für sie