Yes you can allocate jut part of the drive to Peppermint .. but as with ALL partition manipulation, there is a *small* risk of corruption, so back up first .. it's a very small risk, but I'd be remiss if I didn't mention it.
(I'd also recommend running a file system check, and defrag in Windows first)
How much space, depends on how much free space he has on the drive .. and how much extra software he intends to install in Peppermint.