What's wrong with AC if the EM HT red light comes on and air is warm coming from vents?

Is it hot air, or just not cold air? Is the light staying on or flashing?

Im assuming this is a heat pump, correct? I don't get the chance to deal with heat pumps in my area, so I will leave this to the other pros here. But, them ht most likely means emergency heat, and if that light is on steady, then your emergency heat is on, whether it be gas or electric, and will most likely overide the cooling effect of the ac and blow out warm air. I would start looking at stat problems, but lets see what Josh has to say.

Is it hot air, or just not cold air? Is the light staying on or flashing?

It is just warm air. It started doing this a couple of weeks ago and a friend checked with his gages and it said low on freon. He put some in and it did OK until today. I came home and the house was hot and the red light was on. I turned the unit off. The red light stayed on.

Okay, now I would guess that you have a significant leak in your system and the light is somehow telling you that(any book on the system to verify this?) Other than that, like I said, id best leave this to more knowledgeable folks about heat pumps.

Most of the time if there is a problem the light on the stat will flash the fault code on the defrost board. Do you have your owners manual, it will tell you what a steady light means. If your freon has leaked out again already you must have a pretty sizeable leak.