Yes, he could have. The tank and fuel system will have to be drained, filters changed, error codes on ecu reset, only then can damage be assessed. Diesel fuel (and certain diesel oils) contain detergents needed to keep diesel engines, fuel pumps and injectors in good nick, petrol cleans these detergents out. The damage will depend on the distance travelled, the load on the engine (revving / road speed) and the ratio of petrol to diesel. He might be lucky; my daughter was with her first Passat diesel.
I am surprised it happened as I thought that model of Focus (which i own myself) had those special fuel caps whereby the petrol won't fit in a diesel and vice versa - perhaps that's only in the Mk III Focus.