Well, how about a compromise?
Tell her that maybe she could find a part-time job to help support her desire to Pioneer™. Maybe suggest at first that she try Auxiliary Pioneering™ (which has about half the time commitment as a full-time Pioneer™)
You haven't said how old your children are, and whether her JW activities would potentially impact her obligations to them. If it's her taking the children to school, making their lunches and picking them up after school, and helping them with homework, then I'd say to her to go slow.
Understand that within JW culture being a Pioneer™ is a status symbol, and perhaps this is her way of off-setting any disdain or feelings that she is an outsider that she is experiencing by virtue of the fact that she has an Unbelieving™ husband.