I suspected my partner was having an affair when he started being devious with his mobile phone. He eventually admitted it to me, we split up and reunited a few months later. We now have a baby together. It's very hard. Once you have suspected your partner of cheating it never goes away. Your friend has to make a choice. If she really wants to know then she needs to confront him. Or would she rather live with checking his phone in the night and going through his pockets. Having been through it, i now would not wait for evidence, i would speak to him about what is happening and make him realise what it is that he has to lose. Failing that, the devious side of me, would confront the textee and find out how long it has been going on for.
I hope that this all works out for your friend. It is not a nice place to be and she will certainly need friends like you to get her through it.