Which soft the best to unformat HD

[theGate]

Hi mates

A friend of mine format his HD. He format the wrong one !!!
Which software can I use to get the partition back to what it was before.

No new data has been written to the HD only pure format.

I am not a pro in that domain

ps.: I dont want to start a debate just have some idea to what is good to do that

thanks a lot

theGate