Our plan doesn't allow any back cancellations and it really isn't enough money to actively pursue. It's the principal more than anything because we suspected all along that
she didn't plan to return and she just milked the medical and disability coverage.
It shouldn't cost much at all to send it to a collection agency. The fee would come out of what they recover. You may not get the full premium amount back if you go that route but I promise it will prevent this becoming a regular practice. Once we started actually collecting the back premiums, it was amazing how many no longer waited until week 11 to tell us they "couldn't find day care" or "day care arrangements fell through at the last minute".
And this is one good reason to require employees to set up a policy for paying the premium as they go rather than when they return. Either through any pay they receive or through a different method. Now some insurance contracts don't allow for a stop and a re-start, so many employees give a longer grace period. But even in that case, you can require her to pay her part of the premium while she is out.
Paying the usual employee portion is one thing, but collecting the premiums paid on her behalf is another. For my employers, the employee portion was very small as opposed to what the employer paid. I always have charged the regular employee portion so long as the employee was still receiving pay of some kind.