That's a dumb comment. An FA should never say that for a variety of reasons.

First, unless it's a true non-rev, everybody pays for their F/C seats in one way or another. Maybe I paid cash for a true F/C ticket. Maybe I bought a YUP/QUP. Maybe I paid for it with upgrades that I effectively "purchased" across many paid coach tickets in the past. Maybe I paid for it with miles that were purchased across those same past tickets. Even if it was an op-UG, the "payment" to the airline is (a) they were more effectively able to load that airplane, and (b) the positive reinforcement from a CRM perspective that leads me to buy future tickets. So unless she's saying this to an employee-friend of hers, it's both inaccurate and uncalled for.

Second, it's simply bad business to get b*tchy with pax who paid for their F/C seat with a non-cash instrument. It diminishes the overall value of F, and makes people less likely to seek it out on future flights, regardless of their form of payment. I know airlines like to book real cash revenue for their F seats, and they have processes in place to help maximize that by limiting when and how people upgrade. If you jerk me around when I'm here on a leisure trip using miles/certs, I'm less likely to spend extra money to get the QUP fare for my next business trip.