"How do I deal with this once she's no longer lucid enough to sign her return or deal with any other potential tax matters?"

You don't.  You work with what you have. The taxpayer can sign a Form 2848 to you, if not mentally incapacitated.  

IRS would like the friend's PoA to specifically include authority to sign tax returns, but if it doesn't, no one is looking anyway.  If the friend wants to go to court to be appointed conservator, that's the next step.  Which, in most cases can be avoided.