Well, if this ever happens again, go through the purchase process entirely and see if it actually lets you complete the transaction... and then share the results. You can then immediately cancel for a refund to your form of payment under the DOT-mandated 24 hour refund rule.

I am not defending Delta here (because either way this is bad practice), but the reality is that you and your older friend were likely hitting two different cache servers (or even cache keys on the same server).

Going through to the final payment page (and certainly doing the actual payment) triggers a validation of available fare inventory. There's a 99% chance that it would have said "Oops, this fare is no longer available." Again, this is an issue from a bait and switch perspective, but it is probably not individual pricing.