Because presumably at 2 years you see yourself getting married, joining finances, and having kids at some point. It's not overstepping to be involved in long term financial decisions of your partner that will affect you. For years he will have to pay the mortgage, property taxes, repair costs, etc. And while his mom might contribute some money she's saving from not having her own place anymore, it likely won't cover all the costs. And eventually when they want to have kids, are they going to have another house of their own? Because for most people they can't realistically afford the maintenance on two houses. They could also move in to the house they bought for his mom, but does the wife even want to live with the mom when her partner already spends so much time with his mom? Will she just constantly be competing for his time when now he has access to his mom constantly at home and isn't just visiting her. Or what if they want to move to a better neighborhood for the kids, but now they're stuck with the house they already purchased?

There's no shortage of things that could go wrong here. In the end it's his decision because it's his money, but it's stupid to think that large financial decisions like this don't impact your partner, especially if you've been discussing marriage and kids. Living with in laws sounds like a nightmare to me and is a deal breaker I'm always forthcoming about. I could be okay with a partner buying their family a house if they had the income to contribute to our own place as well. But sounds like he left his partner completely out of this decision.