Cold weather starting issues

[LuvmyChevy]

A buddy of mine has issues with his 99 Sierra with starting under cold conditions. If it's plugged in starts no problem. If it's summer no problem. If it's in a garage no problem. But if the temperature even drops a few degrees below 0* Celsius (Canada) which is really not cold at all it won't start. Turns over but won't start. Battery, spark plugs, plug wires, alternator all like new. Any ideas. Confused!