Your buddy is right.

Hammocks pull heat away from the body via convection. Sleeping bags don't work so well in hammocks, as your weight compresses them, and you lose the air pockets that insulate. The hammock and sleeping bag are breathable, so that's a double whammy.

here's a few ideas:

1. a mylar emergency blanket rigged up fairly snug under your hammock (on the outside), and a blanket inside.
2. an inflatable sleeping pad that will fit in your hammock, and a blanket on top.
3. a big rig double bubble visor makes a decent hammock sleep pad in a pinch.
   

e/sp