My grandfather talks poorly about one person one moment and then in good terms the next. I fear he does that to them in person too. Advice?

My grandfather has dementia and can only recall certain memories with certain people. For an example: my grandfather will remember his old neighbor and how he owes him money or screwed him over in the past and then within the same hour talk about how much he misses him and wants to get have a drink with him. He’s gone and visited him once and we dropped him off with him for an hour (we thought they’d hang out a little longer). We aren’t sure of what he said but his neighbor had called us back and told us to come pick him up. He didn’t want to give us a reason why the visit was cut so short. This is where my fear of his memory comes from. I want to put a recorder or something on him sometimes because I want to see if that actually happens since no one ever tells us whenever they hang out with him on their own without me or my mother accompanying him. Do you think that would be in poor taste for me to record his conversations with whoever he goes out to lunch or breakfast with?