You posted in the green card forum so you probably mean Adjustment of Status (Change of Status is for changing to another nonimmigrant status).
If she leaves the U.S. while Adjustment of Status (I-485) is pending, she will automatically abandon it unless she has either Advance Parole, or H or L work visa status which allows immigrant intent (which I assume she doesn't have since you guys are waiting a period of time). She can apply for Advance Parole at the same time as Adjustment of Status, but it takes 3 months to get, unless it is an emergency, in which case she can try to expedite it.
And if she abandons it and tries to re-enter the U.S. on a nonimmigrant status that doesn't allow immigrant intent like tourist or student status, the fact that she has filed Adjustment of Status will make it very likely they will deny her entry. Even if she doesn't apply for Adjustment of Status, and tries to re-enter the U.S., as TripleCitizen said above, the fact she's married to a U.S. citizen may also trigger questions about immigrant intent.