Mom, 88, was just diagnosed with mild unspecified dementia. Any advice?

Not sure what kind of advice you're seeking. Your mom has already been diagnosed with dementia......mild or moderate, all dementia is technically "full blown". Some days are worse than others, sometimes confusion is heightened while other times they're quite lucid. I think you are doing the absolute right thing by placing mom in a nice AL.......hopefully they have a memory care unit or building there as well, because you do not want to move her AGAIN if she progresses to severe dementia. Mostly all the residents in regular AL have some milder level of dementia or cognitive decline, so the caregivers are familiar with the behavior and know how to handle it. The situation you've found sounds perfect and I applaud you doing it sooner rather than later.
Best of luck!

Judy; I'm wondering if you heard this "diagnosis" from the doctor himself or from your mom?

Someone who has been diagnosed with even "mild" dementia should probably not be living unsupervised. Does she live alone?

My reading is that mother has had a proper assessment from a neuropsychiatrist, but meanwhile the neighbour, having seen mother in action, has made what I have to say is his own pretty cheeky assessment.

I forget my house keys about every two to three years. Have done since I first owned any. I'm on two strikes with my current neighbours, who very kindly keep a spare set for me, and am getting quite neurotic about not making it three.

Neighbour doctor may not be completely wrong, but he'd have to define "full blown" and show his reasoning. He may, too, have been speaking for effect, of course, anxious to alert this nice lady's family to the urgency of the situation. I wouldn't ignore reports from him if I were you, though, all the same.

Do you mean that your mom was diagnosed via a neuro psych evaluation with dementia and her new MD neighbor concurs?

I don't care if he is a retired doctor, I wouldn't call an opinion from a neighbour a diagnosis, and certainly not if his only exposure to her is 5 minutes in the hallway.
It certainly wouldn't hurt to take her in to her own physician for a check up though.