I think he has already checked that out
It can take as much as 10-15 mins after touchdown before you are out of the plane, a lot more if there is congestion at the airport gate, that is just like any other airport in the world. Frequently planes land 10-15 mins earlier than scheduled, but you can't bank on that.
After you step out of the plane, one can easily reach the main Flughafen train station in about 10-15 mins (walking, plus the short train ride to baggage collection/exit area).
Add 10-20 mins for baggage collection if you have checked-in luggage.
5-10 mins for buying the train ticket.
The passport check is the only place I would worry (EUs would have it faster though); if you manage to step out of the aircraft fast and rush ahead of others, and if no other plane has landed just before yours, you will be through in less than 10 mins, else it can
take even 30 mins or more.
As you can see there are several sources of variation. It is highly unlikely that you will be unlucky at all places. But you can't be quite sure of making it to the 12:10 train either.
If I were you (I like running, and travel light
), I would go for it.