If a pilot told me something with respect to aviation that happened to him in flight, I would first be inclined to believe him rather than assuming that he must have been mistaken.
He did tell me, and I do believe him. He saw airborne lights that couldn't be explained and which concerned him about the safety of his aircraft. I'm not assuming he's mistaken because there's nothing to be mistaken about in that respect.
I've had a similar occurrence doing the same job, at night, at a neighbouring police unit. I'm merely offering my own experience of seeing metallised plastic foil balloons at night as one possible explanation. They reflect ground lights and look very unusual.
I would rather explain it than let it cause further concern as it's a flight safety issue. If it doesn't fit his experience on the night, so be it. He hasn't rejected the theory as yet.