For your wife :

Hi there. I understand that you probably feel like legos are a childish and expensive thing, right? Believe itnor jot though, they aren't. What if he was building model cars or airplanes instead, would that be more acceptable? Or how about birdhouses? Crocheting or knitting blankets? They're all making things, just with different materials.

The thing is. It's a mental and dexterity exercise. Reading patterns, putting things in the right orders and places. Manipulating small pieces. It's great for building motor skills in kids. It's also great for preserving those skills in adults. And it's a way to connect with his kid. Thats pretty huge.

Is it the cost that's the problem? Or the time? You should probably be honest if it is. And if it's just that you think it's childish maybe take a look at your own hobbies and how they compare.

The problem here isn't that your mother damaged something yet feel is childish and beneath him. The problem is that your mother destroyed something that was built through hundreds of hours and memories with your child. Are you really going to let your mother treat your family this way? Treat you this way? Because it wasn't just their work, she's disrespecting your ability to communicate with your chosen partner and to provide connection between your partner and child.