it's hard to say really.

if the female alter knows them (are you aware of her being with anyone?), then maybe leave a note for her- someone just texted, do you know them?

the other options are you could just ignore the text entirely (delete it from your phone), or you yourself could reply (without giving anything away), I don't have time for this now, I'm busy doing other stuff- or if you're really open and honest, you could reply asking who the person is and why they texted that