Why is my ex texting my roommate/best friend?

Asking again to see if I get more answers. Basically, my ex and I parted 2 weeks ago on good terms because neither one of us - mostly him - doesn't want a relationship right now. It was hard for both of us. My roomie and her boyfriend saw him this weekend and she was surprisingly nice to him - they had a love/hate relationship. Today, her phone starting going nuts so I picked it up and shut it up. Then I looked in her outbox to see if she really sent me msgs she said i wasn't getting. I see messages to my ex. I then had to look in her inbox out of curiousity and saw that he had texted her first. It was a completely random and meaningless message about a food place on campus changing or something - nothing shady because they were never attracted to each other or anything. I feel terrible for looking through her phone but it really bothers me that she didn't tell me. i think she's protecting me because she thinks hes a good guy but says hes too immature for a relationship now.