How to prepare AVCHD on a PC for Mac use later?
Don't do anything to the files, Premiere Pro should be able to handle them. Just copy them to a USB drive and hand them off to your editor. 

NEVER convert your video files prior to editing. You'll lose quality for no reason.

If the Mac won't open them, try another media player such as VLC media player. For a PC, I'd say you may need to install Quicktime as it comes with a codec that may be required, but on a Mac that should be preinstalled anyway. If that doesn't work, update your operating system. Or just try to import them to Premiere, if it doesn't work, update Premiere.