I assume the OUTS are mono. headphones are wired tip = left, ring = right and sleeve = shield.
There's no ring signal if it's a mono out. It's only going to be the tip; left ear.
You need to do one of a couple of things:
connect the outs to a headphone amp, which will combine the left and right signals into one stereo signal.
Go to radio shack and buy a connector that combines two mono signals into one stereo female headphone jack. I don't know if these exist because I'd just solder it myself.
I would reccomend a combination headphone/monitor routing system, like the Mackie Big Knob. That's the ideal solution. because mixing shouldnt' be done on headphones, the outs of the Delta 44 are not meant to power headphones and your friend would have some very useful routing options.
cheers
saburo