I think that this is cutting edge stuff in digital recording. There really isn't a convenient way to do it unless you have DSL or a cable modem.
If you just want your friend to create a few parts for your mix, send a rough mix as MP3 and have them record the other track. If they encode that track to MP3, though, be aware that you are losing sound quality. That can be an issue with lead instruments as it is best to have them sounding as clean as possible.
My guess is that you will probably be better off posting your files to a website rather than emailing them, especially if more than one person is going to contribute. That way they can choose when to download the files (i.e. right before going to bed) and just let it go. I'd hate to be trying to check email at work and have to wait for a huge download.
If you want the person to have the full mix to play with...that gets pretty tough. If you have a CD burner, you'll probably be better off burning the entire song (all the track files and the multitrack song file) to disk and express mailing it. Then your friends can have everything at their disposal, add the new tracks without limitation, and burn the whole thing back to CD and ship it back. Just make sure that you are both using the same multitracking program.
If you must do it online, you may want to try sequencing your song. Record just one verse section and one chorus, etc. Then replay the same files over and over the way your song is set up. That way you'll only have to upload/download a few 15 Meg files rather than a few 65 Meg files. Use it to set up the song structure for the other musicians that are collaborating with you, then replace them with full files (if you like) when they are done adding their parts.
Cubase has something called "InWired" that allows musicians to share multitrack sessions over the internet. I haven't tried it yet, but I think (not sure) that you can get demo versions of all thier stuff and use it for free. I haven't researched it much, yet, and I was wondering if anyone else has.
Good luck!
Chris Shaeffer