What if she moves in with her girlfriend?

A shared-living arrangement, such as roommates or housemates, will not cause a cessation of permanent alimony. A "Kate and Allie" situation, for example, will probably not cause permanent alimony to cease.

Of course, you are free to negotiate a clause in your permanent alimony agreement with your ex-wife that would make any shared-living arrangement a cause for cessation of permanent alimony.

You can insert a clause stating the following:

The wife’s spousal support shall terminate should she enter into a permanent living arrangement with any third party. For the purposes of this agreement, a permanent living arrangement is residing in the same abode with an unrelated third party for a substantially continuous period of (a number of) days.