Mid-60s is very near 55F, the damage point for dieffenbachia. Are you certain the temperature hasn't dipped to that point or lower?

My guess would be sunburn from being too close to a *south* window just after purchasing.

That's possible also.

But depends upon how bright the sun is at this time of year in her zone. Hereabouts, south sun doesn't amount to much right now.

In any event, sounds like battery will make the final diagnosis based upon what s/he knows.

It may be a combination of the 2. Too cool and too bright.

Good point about the light. At this time of year, the sun's angle shifts and reaches into a room to a greater extent than during summer months in northern zones. In another month the current placement may be more appropriate. [November to early February] In late February, take care. The sun will begin to intensify.

If the plant was purchased through a Big Box store it may need slow acclimation to better light and your conditions. Some loss may be inevitable. When I bought plants in big box stores I noticed it took a longer time to acclimate them. No way of knowing how long they have been there. Some plants need 30-50 days to settle in. I went 10 rounds with a Fittonia this summer, similar problem.

Big Box suppliers will grow plants purposely under-acclimated for 'ornamental & toss' home use. Not everyone is a plant person. Many are grown in the lowest light conditions, highest temperature spectrum during the production phase. [Highest yield] Some Green House retailers grow similarly or receive their plants from the same source as a Big Box.
Meg

Thanks for the suggestions. A few brown spots have shown up on a few of the leaves facing the window, so I am thinking it could be sun light. Is that what brown spots on the window facing side indicates?

I am going to keep the temperature the same, but open the blinds only a little bit to let some smaller amounts of light in to see how it reacts.

Regarding the damaged leaves, do they repair themselves or do they need to regrow?

Battery- no they don't self-repair
You may want to rethink your strategy.
It can further lower the temperature

...and what strategy should I rethink? Location? Plant choice?

And what do you mean it can further lower the temperature?

I'm assuming the blinds insulate it a bit.

I'm forced, for space reasons, to keep some of my plants in the windowsill, here. Sometimes it freezes on the inside and I've had my croton and spider plants get frozen to it before, but they've never been that bothered by this for some reason.

A few months later, I am back with a possible explanation. I went into the nursery where I purchased this one and saw the guy who sold it to me. He said the shipment which I bought mine on had the AC in the truck break and it froze everything inside. He didn't have my number to call. I returned it today and he is ordering me a new one, 3 months later, at no charge.

Considering it was sprouting new leaves that looked healthy, I think I found the problem!