Well, this is an awkward situation, I must admit.

Ultimately, the decision should be left to her. While it may be awkward to give your mother orders through battling, she may still wish to battle on your behalf, either to protect you or to satisfy her own needs as a pokémon. Remember, she is currently a pokémon, so what she needs to maintain her mental and physical health may be different from her needs as a human. Moreover, battling is an integral part of the balance between mind and body for most pokémon, so keeping one from fighting when they want to may do more harm than good.

Communication will be highly important in this situation, and as members of the yamask line retain all of their memories from their past lives (including the ability to comprehend language), your mother will quite literally understand what you have to say if you speak to her, even if she doesn’t have the physical ability to respond verbally anymore. Talk to your mother and let her know you’ve figured out who she really is. Outline her options to her (to battle or to become your non-battling companion pokémon) and let her tell you through writing, signing, or any other means what she wants to do.

Is it socially appropriate for her to battle for you, meanwhile? Actually, yes. If you follow Unovan belief (which … would be true in this case, I suppose), then all members of the yamask line are technically spirits of the dead. We haven’t outlawed use of any of these pokémon, even if we have no way of knowing whether or not they were once innocent people, for the reasons mentioned above. Battling, for pokémon, is just another way to maintain their own mental and physical health (as odd as that may sound, considering it’s literal fighting), and humans have simply come to accept this as well as their roles in this process. No one will judge you for having your mother battle—so long as she’s fine with it, anyway.