I will be travelling to Greece soon. My muslim friend has lived there for many years. she tells me she eats the meat, chicken, beef etc that is cooked in restaurants, even though they don’t have a halal certificate. She has saìd that the meat is prepared by Christians therefore it is halal. Is it permissable to eat this meat?

It is not permissible.

https://www.sistani.org/english/qa/01251/