It's a valid emergency, so pressure on the remaining crew is high.
You have to take a decision and as long as its a safe one, continue with the plan. It is very hard to come right away with a suitable alternate at any moment on any route on any given day. Maybe he just remembered weather at STN was good or whatever. Situations like this are not like following a recipe to bake a cake. Any decision which ended up saving the majority of peeps is the best. Captain got lucky. But you can't expect the FO to start cowboying into the unknown being unprepared for the sake of the Captain while in fact he has to take care of a probably almost full cabin, a crew and a multimillion dollar aircraft.
Good job.