My buddies 03 f150 SCREW 4.6 keeps overheating. I replaced the thermostat and Resovoir cap. The rest of the system looks great but he has to keep putting fluid in it. I am boggled. The last thing I can think of is head gasket.

 

That sending unit can act up if that is where the heat up issue is showing. A loose connection on the sending unit can come and go like it is heating up.
Have you actually checked the temp when it heats up to see if in fact it is running hot?
If it is automatic it could be mixing coolant with transmission fluid due to a failed transmission cooling system which would account for the loss of fluid.
I would check the transmission fluid to see if it is too high or looks like it has coolant in it.