Here is the contact info for the ISP. They likley will not help without a court order, but you can always try. They will need the time and date of when the user was online.
person: Karatheodorou Kostas
address: Network Operation and Management Center - NOMC
address: OTEnet SA
address: OTE Building, 3rd floor
address: 47 Mpenaki Str. & Koletti Str.
address: GR 10681, Athens GREECE
phone: +30-210-6151600
fax-no: +30-210-6151900
e-mail: koskar@otenet.gr
nic-hdl: KK656-RIPE
notify: koskar@otenet.gr
mnt-by: OTENET-GR-MNT
changed: koskar@otenet.gr 20040205
source: RIPE
