My fiancé M40 says it’s ok to text “hello beautiful” to his married female friend F40

Me (F40) is engaged to my fiancé (M40). We have been in 10 years relationship. We have had many issues and are trying our best to keep this relationship. Recently I found his message send to a woman who is married with kids. he said she is just an old friend. I never knew about her existence or their friendship until now. In one of the messages he texted her "Hello beautiful!.." | told him that is not appropriate and asking if this is an emotional affairs. He has been denying and keeps saying it's totally normal to say that to an "old friend". They put on each other's messages as well. I told him i would ask Reddit if it is appropriate to text a female friend "Hello Beautiful" if you didn't have feelings for her. So Reddit is it appropriate?? TLDR: my fiancé texted another woman "hello beautiful" and thinks there is nothing wrong with that. I think they are having an affair/ emotional affair but he keeps denying.