Neighbour's boiler flue venting outside my window

I hope this is the right board for this query.
The house next door was sold recently and is being renovated.

We are two semi-ds joined at the garage, but my house has an extension over a converted garage which places a bedroom window above the old garage door.

The neighbour's builder is replacing his boiler and has drilled a hole for the flue just above his garage door, at the furthest point from his main house but right next to our bedroom window.

I am just wondering where I stand on this - is it even dangerous? Should I object or do I have a leg to stand on, since it was our house that extended over the garage, bringing our dwelling closer to his.

Any advice would be greatly appreciated.