If more than 10 years, never serviced, there is a high chance that the orings and gaskets have deteriorated.
Once it comes into contact with a high humidity environment, or constant contact with water, there will be minor water ingress which will cause the condensation problem.
Another possibility:
You said that he brought it to an AD and they said there was a dent in the crystal.
If in this case, it might be the impact to the crystal which caused the dent, at the same time caused misalignment of the crystal, hence it might affect the crystal gasket which in turn affects the waterproofness.
IMHO, it takes quite an impact to dent a Rolex crystal......
Just my 2 cents.
But just like all others said, better bring it to RSC for total overhaul and diagnosis.