You know right now you are telling us you are commiting a workplace crime right.See my follow up post:
People who think it's all good to discuss confidential information such as salaries generally come with other baggage.
If it was a one-off and they were otherwise a strong performer, I'd just say to them something like 'I'll pretend I didn't hear that, but let's not go down that path again'...
this is disgraceful behaviour and there are laws to stop people like you.