There are two steps here - getting her the files, and if they were purchased through itunes music store, activating her copy of itunes.
If she doesn't have the files right now, and there aren't too many, and both your email providers allow attachments of that size & enough space to store them in the inbox, sure. Otherwise, you could burn all the files to CD (the actual files, which should fit at least 100 songs per CD) and mail them to her, which is probably easiest.
Activating her computer is the next step, if they are purchased music - under one of the menus should be "authorize computer." After she has everything working you should probably "deauthorize computer" on your PC, since there's only a limited number of activated computers allowed (unless she's sharing some of the music with you)
Also, finally, this is not something that they officially do, but if you call itunes support and say you lost your purchased music transferring from PC-to-Mac, sometimes they will let you redownload all the purchased songs once.