There's likely no convincing someone that their $20,000 amp or $10,000 DAC sounds no better than a $1,000 or $200 equivalent. Even a blind test in their own room would fail. He'd still find some excuse as to why he should trust his sighted evaluations more.
For me, convincing them is a lost cause. The expectation bias is just too strong. I also take no joy in telling people that they wasted money on an electronic component that makes no difference. I'd really much rather them continue enjoying all that juicy placebo
. I only somewhat debate those folks for the sake of the other - less well informed - folk who may be reading.
I fell victim to these subjective folk early in my audio journey, though luckily it didn't cost me huge money.