On a still day, the wind isn't blowing away the smoke, so as others have said, fresh air is being drawn down the chimney, or it's sinking down on a still day. If you've got a half round drain channel on the chimney, then that suggests no liner, but if there was an issues with the chimney, then they'd be smelling the smoke every time you lit it, so I think the chimney is safe. If you'd just installed the stove, then the blame could lie with you, but having had no problem over the last few years, and only since he's had the DG installed, then that all points to his changes highlighting the issue; but it's a fine line as to who's it is. If you tell him that the only way fresh air can get into his house is down the chimney, he may realise that he's essentially causing the problem, but whether he'll take that on board, is another matter all together.
You could ring HETAS and check for an authorised installer in your area, and they'd have no bias to sell you anything. And if you post a picture of the chimneys, and we can see if there are any sensible alterations you can make to divert the smoke away, but it may not cure the problem if the chimney is his only source of fresh air.
Ok since the cowl has been removed and the smoke from our chimney can go straight up as it was built, He reports "no smell" most nights, except when the air is really quite still.
He says he "Doesn't want to cause a fuss", but despite having the required 2 stage window catches on the windows, refuses to "crack them open" at bed time because he "doesn't like leaving windows open at night."
I have to say, that I'm tempted to write to him formally, recording the issue and my understanding of the cause, whilst the timescale is still clear - What say we ?