Would you be ok with your SO to celebrate a single colleague's bday without you? (More details inside)

Hey ladies,

I'd like to ask your opinion on the following situation and how you would act.

I'm in a relationship with my SO for 3yrs, we live together and engaged. One of his colleagues, a single lady our age, texts him quite often outside of work for non-work related stuff and they even went bouldering together once recently.

She knows he's taken, at least that's what my boyfriend told me, however we're never been introduced to each other and never communicated.

She invited him to celebrate her bday this Saturday evening (with a few other guys like my boyfriend told me), however I'm not sure how I feel about it. Obviously I'm not his mother to tell him who to be friends with and hang out with, however it just feels uncomfortable.

Would you be ok with your boyfriend going to someone's bday in situation like this or it's suspicious?

Thank you for your time in advance:)