In the past I’ve hopped on a call with them and where I’ve asked them to show me it running. When it falls over I say here are the things the system should do, send me a video of the new system doing all of them.

The embarrassment usually shames them into actually checking that the code works.

If it doesn’t then you might have to go to the senior stakeholder and quietly demonstrate that they said it works, but it does not actually work.

You don’t want to get into a situation where “integrate” means write the feature while others get credit.