When you see your friend, make sure to acknowledge their partner.

To be honest, this is a basic courtesy. If you go to say hi to anyone in a group of people, it is polite to greet, or at least acknowledge, everyone there. If you completely ignore that your friend's partner is even there, it will make them feel awkward and neglected. Just say hi.

Don't be overly touchy-feely with your friend, especially around their partner.

Obviously, this mostly applies to friends of the opposite sex (for heterosexual couples). Look, there is nothing wrong with having friends of the opposite sex but just know your boundaries. You may think your friend's partner is being jealous for no reason, but are you doing anything that might make them uncomfortable?

You don't need to always have your arm around them or be leaned up against them. It is really inappropriate to kiss them on the cheek or give them super long hugs, even if that is something you did before they had a partner, and even if it is completely platonic.

You can still hug and be close to your friend, just be respectful of their boundaries. If you don't give their partner any reason to be jealous then they will have no basis to dislike you.

If you invite your friend somewhere, it is polite to also invite their partner.

Even if you assume your friend's partner is going to come, it is nice to make them feel welcomed. And if you don't want their partner to come, make sure they are not together or planning to be together when you invite your friend.

You don't have to always have their partner around, but don't make it a habit of not inviting them. If they don't feel welcomed around their partner's friends, then they probably won't feel as confident in their relationship.

Don't ever bring up your friend's past relationships, especially around their partner.

Even if they are on good terms. Even if you are still friends with their ex. Just don't bring them up. No one wants to hear about their partner's past relationships or flings. It is embarrassing and uncomfortable to have to hear about your partner's exes.

If you are all out together, don't try to separate your friend from their partner.

There is a good chance that if you are out with your friend and their partner, their partner does not know many people there. If that is the case, don't try to separate your friend from their partner.

There may be an exception if their partner has friends around too, or if they are outgoing and can talk to anybody easily, but otherwise, it is really awkward to be in that situation alone. They are with their partner for a reason, and it is nice to make their partner feel included as well.

Just don't make it a habit to always pull your friend away.

Don't put your friend in any awkward or risky situations.

If your friend is a cheater, that is not really any fault of yours. But don't be the friend who is known for putting your partnered friend in risky situations.

There is nothing wrong with going out occasionally with your friends, but it does not need to be a regular occurrence, especially if it makes their partner uncomfortable.

Along the same lines, if you know an ex-partner or fling will be there, you don't need to put your friend in that awkward situation. Just be aware of the situation and how it might make their partner feel.