What's going on, is this flirting?

So my boyfriend of 3 years sent this pic to one of his female colleagues saying 'marry me'. He deleted their conversations and the only thing saved on their snap convo was that (and he hides his phone from me all the time and kicked off big time when he found out I'd been on it)

He knows I don't like her since I've been told by other people who work with her she flirts with everyone and is a slag. I just don't know what to think of this, like should I be mad at this (I am), is this flirting?(I think so)