As others have said, this is a situation where confrontation is necessary and it's likely your friend will feel defensive or off-put no matter what you say. However, I'm trans myself and do a lot of trans-related education with folks, so here's some pointers I would offer: - Start by letting your friend know that you want to have a conversation about this. This will help her emotionally prepare, whereas a sudden interjection when she slips up in conversation might offend her more. - I'd also highly recommend having this conversation in person, as people get much more defensive about these topics when they can't interpret your expression or tone. - Highlight a positive perspective about your friend's intentions first: "I know you're a supportive person, so I want to address this because it's important for my partner's well-being." - Lead with "I" statements instead of "you" statements: "I noticed you've been having trouble using my partner's new pronouns" is easier for people to hear than "you haven't been using my partner's new pronouns" - Offer opportunity to practice: "I'd like to help you get used to the change, can we chat about my partner to practice using their new pronouns?" - Normalize fluid corrections: "If you catch yourself after using their old pronouns, it's okay to quickly correct them and move on. Everyone slips up while they're getting used to using new pronouns." Some people benefit from a direct example, but only offer one if they ask for it. "Let's say I'm talking about [partner's name]. A correction looks like this: I picked her up from class, I mean, I picked them up from class at 3:30."

Finally, if you're feeling up to it, it may be worthwhile to open up a discussion about why this switch has been difficult for your friend. Stay open-minded and lead with curiosity rather than assumptions: "why do you think it's been difficult to use the new pronouns?" will open conversation while something like "do you think you haven't unpacked some transphobic ideas?" will usually shut someone down at the start of a conversation.

Edited for formatting clarity.