Is messaging cheating?

I was just on my SO phone - not snooping just looking at this group chat but a message popped up from another woman saying ‘yeah you?x’ I went on the chat next to him and there seemed to be no conversation. He eventually admitted that he did message her he just deleted his message to her so I wouldn’t find out but it was a big mistake and he don’t know why he did it... I’m so consumed by emotions and I don’t know whether to leave. I don’t want to get into the habit of ‘collecting red flags’ if you know what I mean. Any advice please?❤️

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