Airbrick damp

[tesla6518]

I live in a semi detached 2 storey house. My next door neighbour several years ago built an extension to the rear. When he did this he broke the air brick that transversed the cavity wall leading from outside to the suspended floor sitting room. I have drilled into the cavity wall which has 50mm gap. And using a boroscope you can see the air gusting into the cavity, they have blamed me for damp that they have, but it is also affecting my property. We have been here 32 years and never had a damp problem before. Any advice greatly appreciated

 

[securespark]

Can you post pictures?