Would it be her hanging at his place without her daughter, maybe having a cup of coffee or glass of wine? Or would it be her going somewhere with him (and maybe his kids) as “neighbors” like hiking or to the beach etc?
Would be great to hear your take on that.
I think what you should do first is try to meet the neighbor so you can get a sense of what you are dealing with in regards to their interactions. It may be strictly platonic.