Playing the currency market and timing investments is hard. Most of us can't do this. So you need a plan.
The question is if you will retire to US? If so and long horizon, I would not worry, as your losses adjust with the country you plan to stay in, like an inflation hedge in reverse. . In this case,
Try placing an order for less than the current price, to see if your dad is right. If if fills, then your losses are less.
I choose to keep 80% of my portfolio in stocks where I will retire, not USA,, even if in local currency adjusted indexes tracking the us exchanges, and 20% remains in directly invested us stocks.
Will buying vanguard be hard or expensive if you are not in the USA?
Note...
It can be hard to find a wide selection of blue chips and index funds on some of the non us exchanges.