Thanks for the replies, to answer some of the questions no he never received any warning texts about his data usage until the damage was already done and he got a txt telling him his data usage had been restricted. he rang them as soon as he got home this morning to find out what was happening. Like that he expected a bill of a few hundred euro but not 6 grand!!
Apparently the data roaming cap only applies to EU countries and not Switzerland.
I will let you know when o2 get back to him next week, as I said before there is no way he can pay the bill, he is 22 and lives at home, just finishing off his apprenticeship so literally hasnt a penny!!