Weird response to falkbeer counter-gambit

A friend of mine plays the King's Gambit and is really booked up with it, so I learned the Falkbeer counter-gambit as a response. He fell into the usual traps with it before studying it on his own, but he came up with 3.d4:

I haven't seen anything like this before, and figure there's got to be a good reason. Still, I haven't been able to come up with an easy fix as black. How should I proveed here? I haven't been able to find anything on this line.

Seems to be called the "King's Gambit: Falkbeer Countergambit, Hinrichsen Gambit (C31)"

I have no idea with openings but a person here:

http://www.chess.com/opening/eco/C31_Kings_Gambit_Falkbeer_Countergambit_Hinrichsen_Gambit

suggests f5 is the best response.

Here are a few games by masters to check out for ideas ... seems black grabs a pawn and maintains nice control of the center via ...c5 and the weakened light squares once White plays e5