damp interior brick

I have an unusual problem in my c. 1910 brick 3 story in PS and I’m wondering if anyone has had a similar situation and can offer advice. In 1998, I had to remove the original plaster and expose the brick in the small second floor room above the first floor entry vestibule. Mold and mildew had formed in one spot, about 6 feet from the front of the building and 6 – 8 feet from the floor. I inspected the 3rd floor tenant’s apt. above and their wall had no signs of water damage. The very small amount of moisture evaporates now that the brick is exposed (and the mold problem is gone), but it has caused some of the mortar to effluoresce and some of the bricks to crumble. Furthermore, I often smell a musty brick odor in the room. My immediate neighbor is slightly up hill from me, and I believe the problem may be inside his chimney, since the moisture is so far from the roof and his chimney runs very close to where the damaged brick is prevalent. Furthermore, I have had the roof repaired, the cornice molding repainted, and the pointing inspected to make sure that water was not getting in anywhere else. My neighbor also has a chimney rain cap. So where does the moisture come from? Could it be that there is a great deal of condensation in his chimney? If so, why? I am worried that he may have to replace his boiler and reline his chimney, which could be hard to get someone to do amenably. Any thoughts?
Thanks, Roger