If this was only the first time, then I would forgive him and trust him to make the right decisions. If it happens again or has already happened before then you're going to need to talk to him about this.
Oh Im sure he will text her or any other girl again. He'll be more smarter next time. I wouldn't keep bringing it up to him.whatever he's doing will eventually come out. So don't sweat it!!! Text a guy and see if he likes the feeling
Theres nothing wrong with him txtng other girls. The only time you should trip is when hes flirting with them or sending them winky faces. If hes just talking to her just as freinds dont worry about it. It will just be unecessary stress on you. Give him freedom thats what builds up trust.
When a woman is able to grow and mature to a point where she is no longer a slave to her own worries, insecurities and fears, then she can finally look past her own wants/desires/fears and see the man?s.