Help please! 06 Ford Focus - heater blows HOT at idle, COLD while driving!

A few days ago, I took my
dad's 2006 Ford Focus 4
hours away on a trip. About
half way here the heater
started only blowing
lukewarm (at best) air even
though the car was obviously
up to operating temperature. I
stopped at a gas station to
warm up and noticed that, at
idle, it blew hot air like it
should. Once I started driving
again.. Coldish air. I got to my
destination and the next
morning I had a friend look at
the car, he discovered that the
coolant was frozen.
(awesome, right?) He then got
the car thoroughly warmed up,
flushed radiator, made sure
there wasn't a clog in the
heater core, and replaced the
coolant with new 50/50
antifreeze.
.... Still. No. Heat.
Both hoses going to the
heater core are warm after
running, though the outlet
hose is noticeably less warm
which I read it's normal as the
coolant cools while running
through the core.
There's no clogs, that he's
found. No leaks. No bad
hoses. No smells. No weird
fog. No heat.
The temperature gauge inside
the car will go from operating
temperature, too just barely
above cold and back and forth  
while driving.
We bought a thermostat but
not sure that's the issue...?
It is very cold outside, with
windchills at about -25°F to
-35°F... Could it just be too
cold, in general?? I've been
told to put cardboard inside
the grill to keep the engine
warmer while driving? Idk...
Help please, I'm stuck four
hours from home, I need to
get back to work!
TIA!