How do you respond if you found out that your bf is having an affair?

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I got a text message for my bf on my cellphone and it came from a woman...i replied her back asking where she works but it seems like she got mad at me and i dont know the reason. In fact i suppose to be the one who react that way because of her txt messages demanding my bf to meet with her. do i need to ignore them to make everything fine or let that woman know that i am his girlfriend?