Even if he was a hair under the 15 minute limit, the GA had already had time to realize he wasn't there, process an upgrade for someone else, go find them and move them (if already in board), or board them, etc. Seems pretty plausible that this happened well in advance of 15 minutes ("we didn't think you'd make it").
Rules are rules, and if he turned up at 14 minutes there will likely be no recourse (even though it clearly wasn't your fault—sorry OP). But hiding behind the rules leaves a bad taste in my mouth: I bet the result on the ground would have been identical if he'd shown up at 16 minutes. UP fares are sleazy, as is the general indifference we often hear about directed towards paid First passengers (at least domestically) on here. I think it's a real shame.