Sold ticket on ticketswap, I think it's a buyer scam

My friend bought tickets to a concert and decided to sell them on ticketswap, because she got tickets with better seats.

She sold them and the buyer received the pdf tickets. But now ticketswap messaged saying the tickets aren't valid, because it has her name on it. The tickets are legit and were bought in an official site.

Ticketswap is demanding the seller to refund the money, while the buyer already has the tickets and will most likely will use them, because they don't ask for ID to confirm the name on the tickets.

What can she do about this? will she loose the money?

Is it possible to see if the tickets were used to enter the concert? (so she has a proof that the buyer used them).

Thank you in advance.

Edit: the concert is in Spain