Need relationship advice!

Me and my boyfriend have been together for about 9 months. He knows this girl from work and I've seen them texting before and told him how I wasn't comfortable with it. He claimed they were just friends and she knows he has a girlfriend, but he said he'd stop. Recently I was using his phone for something and I saw her text thread was on Do not disturb (meaning her messages wouldn't pop up in his notifications) and he was texting her, but I didn't go through the texts. I'm not sure how to handle the situation considering I already confronted him once. What would y'all do?