I've had a bit of a nightmare with my XL2720Z and an Asus 1080ti with getting it running properly at 144hz. I'm currently using a displayport cable but it randomly says it's out of range, even though if I flip the screen to 60hz and back to 144hz it will work again for a few days.
Anyway I think you would need to use an active adapter DP to DVI (Dual Link) but from what I've been told (not sure if true) is they can be quite expensive and might only top out at 120hz.
A new monitor would be a solution, but possibly a pricey one depending on what he wanted to go for.