How would you feel if your best friend doesn’t allow you to bring your partner to her birthday party?

Long story short, a friend of mine has a bff who’s boyfriend thats not necessarily liked due to past actions and current actions in said relationship and didn’t want him at her birthday party. Now let’s call the bff M. M wants her boyfriend to come the party and is upset she can’t bring him. M went to the party but is still upset at our mutual friend.
How would you feel if your best friend doesn’t allow you to bring your partner to her birthday party?
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