
Quoting
llworking
In the case of a parent who is truly awol from the children's lives it might be ok, if a parent is deceased it is probably ok. However, in most cases it is extremely inappropriate and disrespectful of the actual parent/child relationship and if they happen to be in and out of court, it can cause serious problems for the parent who is allowing it to happen.
Where it is the most inappropriate is when the person being called mom or dad is not even married to one of the child's actual parents. So no, "if they want to call her mom they can" is honestly not an appropriate answer.