I'd ask him to specifically list the reasons the HK is (supposedly) better than the Glock.
Throw out the one about it "being a HK". If that was a real reason, then there would be no need for other guns on the market.
After he lists the reasons, give each one an actual test or at least a BS smell-test. Trust your gut. Ask questions.
If any of the reasons seem legitimate, then ask yourself if those reasons are worth the price difference TO YOU. They may be worth it to him, and not worth it to me, but that doesn't matter; you should only pay for things YOU can use. Example: long-term durability is often a selling point for certain guns, but if it was your intention to shoot one magazine of ammo through the gun to function-test it and then drop it in the nightstand forever, long-term durability should not really be worth anything to you.
I'll mention that parts availability for HK weapons sucks, and when you CAN find them, they're always expensive. Spare magazines also usually cost more, but not always. Although they MAY be very accurate, many folks do not personally shoot HKs all that well (based on my experience), although I cannot say why with any level of confidence. Glocks are easy to work on, parts are plentiful and cheap, and enough folks shoot them well in competitions like IDPA that a person can't really argue that they are hard to shoot well. If any of this is important to you, take it into account when you make your decision. If not, or if you don't think it's inaccurate or might be biased, then don't use it in your decision-making process.