Legal Question in Family Law in Massachusetts

Both you and your ex-wife must obey the court order. If the court order gives your ex-wife the ability to control who is with you when you are parenting your child then she can tell you your girlfriend can't be around. However, if the order is silent on this matter, then she has no right to tell you who can be present when you have your son.

On the other other, if your girlfriend is harmful to the child, then your ex-wife can probably go to court to get an order to keep her away.

If it's not addressed on a court order, there's no restraining order, and she's not a danger to the child, you should be fine. It will be difficult for her to convince a judge to take action on that issue.