Son’s first girlfriend!

Hi Ladies,
I need advice on how to speak to my 13 year old son about his new girlfriend and some of the messages she sends him. She says that she is going to hurt herself all the time and seems to be fishing for him to reply “no don’t do it” etc... I am pretty certain she isn’t serious with any of her threats, but what do I tell him to do/say when she says these things.
I feel torn because I don’t want to say that I snooped through his messages (I have told him in the past that I am entitled to look through his phone at any time if I feel there is a valid reason!), but I feel like these messages were very private and so feel like I have over stepped. Also some of the things they are saying to each other a pretty full on and I want to address that with him too.