For a downgrade she would get 75% refund of the fare calculated for the downgraded sector. If it was a cheap ticket it might be better to try to agree with the check-in staff for a lumpsum comp. for the downgrade rather than trying to rely on the EU Reg, 261/04 comp. Sorry I'm not aware of the curent standard downgrade amounts for C to PE or Y but others may chime in on this.