After reinstalling windows you have a 25/1000 (= 2,5%) chance of your file being overwritten (assuming 1 TB hard drive) => 97,5% chance it still exists.
Thats just a theoretical way of calculating the probability.
You can't calculate the probability of the file still being there.
It all depends on how full his disk was before he formatted, what filesystem was in use, what happend after he reinstalled, etc...
If his disk was 90% full before he got into bitcoin, his wallet.dat file might be more at the end of the disk. Formatting the disk and reinstalling windows will mainly write to the beginning of the disk, leaving a good chance for the wallet.dat file to still be recoverable.
If he got into bitcoin early, when his disk was only 2,5% full, his wallet.dat file is probably more at the beginning of the disk and a reinstall might have overwritten all of it.
Updates to his wallet.dat file doesn't even necessary make a difference because of the way NTFS/FAT filesystems work.
Those filesystem reallocate a file to different sectors if they need more space but it can even be spread over multiple places on the disk. But if the wallet.dat file hasn't change much in size there's a good chance the file is lost forever after reinstalling windows.
But as I said there's no real way for calculating the probability.
You could try pywallet for scanning your disk from beginning to the end. If you need more help with that, feel free to contact me so I can help you through it step-by-step
