Yikes. My buddy did the same thing last year on a rental flip. Saved like twelve bucks on the valve. Three days later he gets a call from the tenant, water pouring down the hallway. Turns out the cheap ones have plastic guts that crack under normal pressure. He ended up paying for new floors plus the drywall. That twelve bucks cost him almost four grand. Not worth it ever.
Four grand for a twelve dollar part is the kind of math that keeps you up at night. I feel for your buddy, that call from the tenant must have made his stomach drop. I got lucky on my flood job, the damage was mostly contained to the laundry room and the homeowner caught it early. Still had to rip out baseboards and run fans for two days. Those plastic guts everyone trusts are a joke, they crack just from normal water hammer. Cheap out on a valve and you basically sign up for a remodel you didn't plan.