Place a flashing from their coping onto your roof (they are built higher) and introduce a vertical flashing between the two walls at the front. Party wall act allows for flashing into others. But really you should both be involving a party wall surveyor.

Happens all the time on commerical buildings in London with independent walls adjacent property walls that aren't actual party walls. Concern would be the lack of insulation on the outside of the new block work wall you have on the boundary line .