I have a @mac.com email from the early days but when they starting charging for MobileME, I let it lapse and stopped using it for email (but still kept it as my AppleID address).
Once iCloud came around, I looked if I could revive it but no dice. However, in just the last couple of months I’ve noticed emails coming into my @icloud.com account that are addressed to my @mac.com address. I did a test email and found that it’s now working as an alias to my @icloud.com account.
It might be worth checking your friend’s iCloud account to see if there’s some way of turning on the alias feature for the @mac.com address. (Or reach out to Apple support and see if they can help - even though I’m well out of warranty/AppleCare, Apple techs went above and beyond to see if they could diagnose an issue I was having with Apple Mail.)