Well she's not going to be getting MCE seats for free unless and until that's all that's available. Even if it's middle seat in last row and a dozen MCE seats, she'll get the middle seat in the back. That gamble probably isn't worth it, IMO.

Preferred seats should start coming more available at T-24 if not a little earlier; but I think sometimes those are saved for elites or non-elites willing to pay $ (at least until airport checkin).

I really see no disadvantage to her retaining her current seat assignment. Just keep checking back frequently, especially starting a little before OLCI opens. I think even if you could remove the seat assignment (which would most likely require agent intervention), there's a chance the system would re-assign it anyway.