So - is he being jammed/blocked? -it's a detached house in a rural location, however there is a rumour that a neighbour 200yds away had a lot of BT Business vans around some time ago.
No, he's not, and jamming wireless signals, whether regulated (eg mobile phones) or unregulated (eg WiFi) is an offence. 150m is about/possibly above the limit for WiFi from an internal domestic router, so it's in the realm of 007 to imagine that his neighbour is disrupting it.
Have you tested both laptops sitting next to, but connected by WiFi to both routers, in both locations? Not all routers like all laptops, and no router lasts forever - they do die.
Have you tried changing the channel on the router? You have channels 1-13 ISTR, and it's possible, and far more likely, that something in his house is interfering with the channel you have set currently.