Can you try to explain this for me please?

A woman I know has met a guy through a dating site, however, she threw a tantrum when she learned that he'd met other women from the site (before she'd spoken to him). Apparently he has 'completely broken her trust'. She will not shut the hell up about it (although she's been told to, on more than one occasion). She's not my friend, but part of my social group due to her knowing friends of mine, so avoiding hearing it is difficult at times.