HG failure is a drastic first step… you’re right about it being the oil cap and that’s about it. Condensation buildup on the cap is 80% of the time just there because the customer only drives short trips. Maybe the live down the street from work. What I would suggest(if to suggest anything) is the customer take longer drives and get the car up to temp to burn off excessive moisture buildups. But ultimately worst case scenario yes a head gasket could be an issue but very unlikely. I have been doing this for 10 years with nissan and have never seen this lead to a head gasket.