So your partner's friend is a bore. Their behaviour stinks, their jokes are awful and they know far too much about your relationship for your comfort.
The worst part is that they are definitely not on your "side". Their loyalty follows their friendship. When it comes to relationship tensions, someone being on your partner's "side" can sometimes feel like someone "against" you.
You probably like to talk about your relationship with someone you trust, and you might feel a bit vulnerable with your partner's choice of confidant. Someone you haven't chosen gets to know all sorts of personal things about you, and that may not always show you in a very good light.
Partner's friends tend to be more concerned with how well you treat their friend than with what works for you. It can feel like they expect more of you than your partner does, and they may not be as sympathetic to your perspective as your partner might be.
If it's a really long standing friendship you can end up feeling left out while they reminisce about old times. You might feel that your ability to know your partner well and to offer them intimate companionship and support is challenged or diminished.
So what do you do? If you end up in the uncomfortable position of feeling exposed or excluded or in some way threatened by your partner's close friendships, how do you handle the situation?
The first thing to remember is that your issue is usually with your partner and not with their friend. If your partner talks or behaves in ways you don't like, they are responsible for that behaviour, not the people who are with them when they do it.
You might want to pick a time when you're both feeling good to discuss the behaviour you're uncomfortable about and to consider some alternatives that are okay for both of you.
Another approach is to consider how the conversations between your partner and their friend might actually benefit your relationship. Do they help your partner to let off steam? Do they help your partner to clarify their concerns, or frustrations or distress? Do they allow your partner to express parts of themselves that matter to them but don't fit in with you?
Sometimes it's hard to tell the difference between feeling vulnerable and actually being threatened. Maybe your attitude to the friendship presents you with more of a problem than the friendship itself. Maybe you could think about ways of developing more confidence in yourself, your partner and your relationship.
If you find you are really unhappy about what your partner shares with their friend, maybe you could talk about that together. Perhaps you can jointly agree some guidelines that will reduce your anxiety without intruding on your partner's friendship.
If you both understand the value of the friendship and the conversations to your partner and the source of your anxiety then you have a better chance of reducing the tension.
You need a bit of give and take here. Relationships usually struggle if the people in them are cut off from outside friendships and activities. You probably both need more stimulation than that.
On the other hand, ignoring a partner's distress, even if it seems unreasonable, doesn't help a relationship either. Like most things in relationships, it's a balancing act.