I think your friend is trying to apply a questionable solution to solve the wrong problem.
I'm playing PUBG with a 280X, it works fine. If it's crashing there's clearly a problem, but how do you know that a cooling block is the right solution?
On top of that, a 280X sits somewhere between a RX 560 and RX 570 performance wise, spending significant money on a water cooling block for what is now an entry level gaming card is not a great way to spend money.
Is your friend monitoring temps while gaming (in AMD Catalyst, Afterburner or HW Monitor)?
Are you sure temps are the issue?
If they are, check the cooler for dust buildup and check the GPU fans are actually working. The cooling solution on the card should work just fine, there's no need to put in on water.
If the temps are okay, then obviously a water block won't make any difference whatsoever.