1: Buying a high frequency monitor will not make you better at mania. (see: every thread discussing mechanical keyboards, this is exactly the same)
2: High frequency monitors are still more expensive.
3: The only benefit from high frequency monitors is a reduction in ghosting at high scroll rates, which is a minor advantage at best, since it's been proven that top tier players do not require this to read high scroll rates and complicated patterns where ghosting could actually make a difference.
$: In conclusion, it's not really worth it, unless you're rich enough to not care, or have other reasons for buying a high frequency monitor (or you just want one, regardless of any other reason).
Can he still chose to buy a high frequency monitor? Sure, it's his money, and I'm not gonna say it's completely useless. The question is whether or not the advantage is worth the cost, and in my analysis it's not. Just like mechanical keyboards. There are benefits, but those benefits are not enough to be worth the price of using them. It comes down to preferences, rather than cost effectiveness. I can't speak for his preferences, but I can speak for the cost effectiveness of high frequency monitors.