Ex harrassing partner

My partners ex is harrassing him with text messages rubbing it in that she ran off with his kids to move in with another man that she's only known for a week and sending him pictures of him with the kids saying my happy family now, thanks you have done me a favour etc. She agreed to let him see the kids through her lawyer by supervised visits and now she'd rather send him upsetting pictures revealing his kids with a complete stranger than let him see his own kids and the kids look very unhappy in the picture. She took the kids away from an area that they grew up in, went to school and had all their friends and family, just to move in with a complete stranger who could be capable of anything. She can't look after herself never mind 2 kids and can't control her temper, that she takes it out on the kids and even hits them so if he's a child abuser, she'd just stand back and let him do anything to the weans. They're not even my weans and I'm concerned because I know my partner is a good father and I'm looking forward to the day we have kids of our own. I've seen her ridiculous behaviour: she won't cook, clean, do the washing or wash dishes so the poor weans are in for a really bad time, she just sits on the couch all day eating non stop so no surprise she's well overweight. My partner has been warned not to track his children down to take them back cause he can get arrested but I think he should have the right to because he is concerned for their safety and wants them back out of the danger that she'll have them in. He's a good father and she's a bad mother. I'm just wondering what anyone else would do in this situation. Thanks to anyone who answers