This has been tested in the courts but I am not 100% sure of the latest answer. A lot of the employers we work with make their employers sign this sort of declaration (to try to stop them either wasting the course fee, or upping sticks for a better job once they have the qualification) but I'd always understood that it was more of a moral obligation that a legal one.
I'd suggest she asks someone like ACAS (which is on-line and free) or looks at sites like this one for precedents
http://www.i-resign.c.../new_topic.asp?t=1355
One option we sometimes suggest is that we look at how long the person still has to go on a course (e.g. she's three-quarters through) and ask the new employer to pay the remaining 25% back to the employer who paid for it - after all, the paying employer has had the benefit of your wife's new knowledge for the time she's been studying.