Frozen drain plug

try using a pair of vice grips

Carefully file the rounded edges square as much as possible. Select a socket that seems slightly too small but sort of wants to go on. Gently tap that socket on with a hammer. Then try to loosen the bolt being sure to keep the drive square and holding it tight to the bolt. If the bolt is metric, which it probably is, then the tightest fitting socket may be an SAE size.

PS> Have a replacement bolt available and perhaps an oversize one for if the threads strip and even a backup universal rubber plug if nothing else seals it. And don't let the wife change oil next time.

Try a 6 point 1/2 inch wrench

...and tap straight onto the wrench/nut as you exert turning force--a little shock from the tapping often works wonders. Also, might help to attempt this while engine is HOT. s