Think you should read Kelly's post and you should keep quiet.

Your mom should have never told you. That is a violation of privacy and maybe against the law. It doesn’t matter if you all know each other and she likes to gossip. That is a problem on its own. Now you are put in a situation where if you say something to your friend your mom and his girlfriend will have a problem with you and if you don’t you are not being a very good friend. I would just act like your mom never told you this and tell your mom that what she did is a privacy violation. It isn’t right but I wouldn’t want to get my mom in trouble and also my mom and my friend’s girlfriend to have a problem with me A LOT more than not being a good friend to my friend. You are going to now have to decide what matters more to you.

Do you have any idea about her timeline?  If she's already pulled her daughter out of school, I'd think her departure is imminent.  Does your friend know she's done this or has he said anything about it?  This is a pretty big clue about what she's up to.

I don't think you should come out and tell him, mainly because your mom shouldn't have repeated what the gf said to her.  But I'd definitely want to hint around, or even stage something.  Do you have a valid reason for driving by their place?  Try it and if you see a moving van, call him.  Or if he mentions something weird, ask him point blank what he makes of that. 

Finally, ignore the answers about kidnapping and calling the cops.  That is nonsense.  In the US, it's not illegal for either parent to leave with their kids.  What law are they breaking?  The only time kidnapping comes into play is if there's a court order in place, and one parent violates it by taking the kids when they don't have legal custody.  Or not returning them when they should.    None of that applies here.

No, the relationship of others doesn't concern you.

If your mom is a principal, ask her if she is familiar with FERPA?   FERPA is the education equivalent to HIPAA and it's for minors in education whether they are in public, private or charter schools.  Your mom telling you anything about a student you are not the guardian of is a FERPA violation, your mom can be fined and terminated for it and she should be.  If your friends GF wants to make a big deal about it, she can take it to the school board and file a complaint with the state and federal government about it.

Private schools aren't exempt from FERPA because they still receive government funding from states and the federal government (for federal usually in the form of grants).  Your mom's inability to not take student privacy serious can result in the school or district to lose funding.

https://www2.ed.gov/policy/gen/guid/fpco/ferpa/ind...

Tell your friend, friends should count on each other. If he finds out you new, that might end your friendship

The legal issue depends on multiple factors, and i dont know what differs in your country, but in England it concerns whether they are married and when the child was born in terms of a parent taking a child away from the family home. Especially if the parent that is leaving with the child is the mother. Its ****, but it's the system. The police will not get involved as it is a civil matter, not kidnapping, so it would be down to the parent left behind to instigate court proceedings to gain access to the child. As i say, that is in England.

As for the heartbreak, it's only a decision you can make in terms of how well you know your friend and how they will likely react. Surely he knows his child has been taken out of the school? Is the mom still dressing the kid in the wrong uniform and pretending to take them to school each day? And how old is the child not to say anything to the dad? Such a drastic manoeuvre must have a reason, and unfortunately we don't know what happens behind closed doors to cause someone to want to run away in secret. It may be worth speaking to the girlfriend in private, explaining that you're aware of the situation and feel uncomfortable just watching from the sidelines as it all falls apart.

I wouldn't. Unless this friend is an idiot they know they can call the police to report the kidnapping if this woman actually does abscond with the kids. 

If its his child and she leaves, it is considered kidnapping. he needs to know what she is up to. He will need a good lawyer to stop her. He has a right to see that child, and i she leaves she needs his permission.

Is your friend the father of the child?  If so, does he have any court ordered visitation or par time custody?  If he does then the mother can not legally take the child out of the area covered by the court without a hearing and the father's consent.  

Butt out. 

Your mother needs to learn when to keep her yap shut.

What exactly do you think your friend would do with the information anyways?   She isn't under any obligation to stay with him and the courts can work out a parenting plan.