hmmmmm........ why not just get a new and nice basic diver that can withstand his usage now and buy the grail piece later.
If his taste in 'grail' pieces for heirloom is conventionally (or not) like mine e.g. something from the holy trinity, ALS, I would encourage him to pull the trigger if, in order of importance:
1. Affordability is not an issue (well not in the iceflight sense, but I would measure it as still earning a healthy income and having enough finances for a reasonable emergency after the purchase).
2. He is pretty confident and set in his specific choice of the grail piece already.
3. No family resistance (if applicable)
Prices of luxury watches will generally just climb upwards over time, so getting it as soon as you can afford and know what you want makes sense to me.
And may I add: can consider reaching out to jase07.. he seems pretty resourceful in watches in the grail segment, based on his posts.