Why in God's green Earth would a guy do something that stupid? Sounds like he is TRYING to make you jealous. Just saying...
Text him back and just tell him 'hope you have a good time' with no further response at all... leave him wondering WTH?
If you know them to be friends..it doesn't sound like cause for alarm...unless he's the sort that tells you things because he wants you to be jealous (and if he were that sort I'd say stay away from him in general)
The reality is, trust is a choice. You have to decide if this boy is worth your trust...if he isn't it doesn't matter what he does because he's not for you. If he's worth your trust...then you have to consider getting more info about their relationship or simply taking him to have good intentions in general.
My advice would be to do nothing right now..and just ask him who she is later if you still think it's significant. He may volunteer the information eventually anyway considering you only know what you do because he told you.
find something else to do
If you trust him and they are truly just friends then let them visit and leave it at that.... he cared enough to call you to let you know so I doubt he is trying to hide any thing.
at least he texted you and told you
but talk to him about your feeling of you not being the girl thats with him
he texted you ... i imagine its ok for you to text back
and if you two are in a relationship and its not ok to text him ... then you can worry
of course, its within your right, and if he cant tell you then somethings going on
Go supervise lol.