Unlike most buffs you don't need to worry about INT when it comes to stone all. That's the good thing about it, it will still lasts 4 turns and protect against STR attack regardless of what your INT is. BUT if you want her to do good damage when she does attack then adding INT would be helpful.

So maybe try going for both good CON and INT. If you don't think she will ever be useful as an attacker and you don't plan to use a brooch on her then choose con so she lasts longer in fights.