He cheated when.with his ex

Hi all. I have been with somebody the last 6 months. He told me his ex in his previous 15 year relationship broke up with him out of nowhere. He reckoned she changed and became distant after she had a miscarraige. He recently told me that she accused him of cheating after she found a message in his phone to a girl that they both knew. He denies cheating and says it was innocent. i dont fully believe this. Im worried now that this is a red flag for me. The girl he was accused of cheating with is related to his friends and she would be around a lot in the future. I am in my forties now and i have a child. I cant get another relationship wrong. I am looking for a high value relationship now. I will also add that i am insecure and have been working on this with a counsellor the past 6 months. Im not sure if i need to be worried about this. In a way is it none of my business what happened in the past?