A man sends a message to your girlfriend saying "good night, dear". Is this a joke?

Maybe it's just a little ambiguous.

But what is certain is that this man wants to say something to your girlfriend with this.

Dig deeper, two people may say that it is normal for friends to say dear now.

If you don't dig deeper, he'll think he said something, dear, as if he said something.

So, you must lose.

I suggest you have a good talk with your girlfriend and let her have less contact with him as a way to respect your feelings.