Custodial Parent Using Cocaine

My question involves a child custody case from the State of: Virginia
My fiance's ex-wife has physical custody of their 2 little girls, they share legal custody. Just last night we found out that she was fired from her job last month for multiple positive drug tests (beginning in Mar 08--cocaine and marijuana). She even tried to tamper w/ her urine to avoid a positive. In addition, the oldest child has reported some neglect issues. Her former boss called CPS and they said there was nothing they could do since it was a first complaint. My fiance plans to file for custody this coming week. The ex doesn't know that he knows anything about the drugs yet. Is there is possibility for a custody reversal given the circumstances? She financially is unable to provide for her children, on top of her illegal drug use. What else do we do?

Your fiancé brings his action for modification of custody, and presents his evidence to the court.