Internal Wall Crack Repair

Have received a request from an owner  for repair of cracks on their internal wall. The building is double brick with a cavity.

The builder talks about moisture build up due to poor ventilation in the subfloor and lack of vents in general. Speculates that the grass section area on the external wall may have become drenched with rain overtime(like 50 years worth!) and affected the wall etc.

I  would have thought the internal wall is the responsibility of the owner.

Since the building was built to a standard when it was constructed, is the OC liable to install further vents etc?

His other reasons also seem speculation rather than fact.