I will start by saying I am definitely not a lawyer, and not well versed in EU laws. It would be surprising to me if this isn't opening Ryanair up to liability. As far as I can tell, the chargeback was essentially a mediation handled by the bank, the bank ruled in favor of customers, and Ryanair simply decided that it didn't like the decision and will continue to collect.

They would probably be in the right if they proactively contacted the customer they did not agree with the chargeback decision and the customer was no longer allowed to have tickets in their name until the debt was satisfied. Neglecting to contact the customer until after the customer bought another ticket, then likely holding that ticket in purgatory until the customer pays the additional amount is the issue. I'll go out on a limb and guess they said either they won't refund the new ticket or are being vague about a refund if the customer refuses to pay up.

If there was a court ruling the customer this money, Ryanair would probably be fine. If Ryanair contacted the customer and said they could not purchase any more tickets or fly on the airline until this is settled, they would probably be fine. As it stands, this sounds completely ridiculous and wouldn't surprise me if there is some consumer protection /aviation law being violated in the process.