If friend is using the Mac's own "Mail" application, there is an option to send "Windows Friendly Attachments", which might help with the problem.
In Mail's menus, go to "Edit > Attachments > Always Send Windows Friendly Attachments".
Having said that, I've never had a problem transferring graphics files between Mac and Windows. I've also just emailed a couple of files, a JPG and a GIF, from Mac's "Mail" application, retrieved them with a PC and there was no problem saving and opening them. This was *without* enabling the Send Windows Friendly Attachments option.
If friend is using another email program, it may need to be configured differently, for example it may not be using MIME to encode the attachments.