Reading into the wiki:
"Only places that provide a proper choice for an option should be tagged yes. For instance, a restaurant with vegetarian starters but no vegetarian main course is to be tagged no."

If I extend this logic to this case, that would mean the value should be 'no' as no consistent vegetarian options exist.

Source: https://wiki.openstreetmap.org/wiki/Key:diet:*

Marking this place as diet:vegetarian=no

Closing, resolved in changeset/178308691

If you disagree with my conclusion, please feel free to re-open this note.