I agree with Taxagent...nothing your wife did was custodial interference.
If the wife was given temp court orders naming her the custodial parent and she asked your wife to babysit, that is perfectly fine.
If there are no court orders and the wife asked your wife to watch the kids, that too is perfectly fine.
If the Dad had custody and it was his custodial time and your wife had the children and refused to return the children to him, that is a different story but not the one you told. The guy sounds like a jerk.