My girlfriend likes to chew on chocolate wrapping (tin foil) Is this bad for her teeth? She says it feels nice.

No. This is not ad for her teeth since the wrapping is soft.
Stop. This should not really create a large problem, but constant chewing on the wrappers cannot do the muscles of the face, bite and jaw any good. I guess it might also depend upon how much chocolate she chews, which is another nutrition issue.
Not smart. Can cause untoward wear. Danger of small sharp pieces being swallowed and injuring inside of GI tract.