Losing photoshop would be painful .. gimp is great but it is still no photoshop.  You could probably run those under wine though, so she could likely keep photoshop and Painter.  Always better to leave the dark side, fully, but hey.. if you can get her onto Linux with the exception of one or two apps, that's gotta be considered a win.

Maybe just remind her she's using the OS of the Dark Lords .. the OS of Mordor .. the OS of the digital slave.  Maybe over time, you can wear her down!