My buddy has a 2007 FX4. When he stops at a stop sign or red light the AC goes from blowing cold to blowing hot. Then a few min later it will start blowing cold again. Any ideas?

 

Air flow through the radiator is my first thought. My A/C does not blow as cold when stopped as when I am driving.

 

I have heard this from several people. I have an 07 lariat 4x4. I have sat at idle for 30 minutes at a time. A/c always works well.

 

A/c performance will reduce somewhat at a stop/idle due to a lack of subcooling in the condenser. The heat from the radiator will conduct to the condenser if your fan isn't moving enough air. I'd imagine your fan isn't engaging, or isn't receiving a signal from the PCM to engage.
You should have atleast a 20 degree temp drop from incoming air to the evaporator to the air out of the vents.