How to tell officiant (friend) they won't be marrying us?

Be honest. You don't need to elaborate anything. If the friendship has run its course, then you shouldn't feel guilty. But then I also wouldn't invite them as a guest either because that is reserved for your closest friends/family who have not compromised your relationship with them.

Agree- just be honest. This issue is one of the main reasons why we opted to hire someone to officiate our wedding. Sometimes friends & family just shouldn’t be asked..,

Agree with honesty here, although I do think it will be a lot harder to replace one friend with another vs replace a friend with a pro. Expect that your friend will not be happy about your decision and that it will ruin your friendship entirely (which already sounds strained, so maybe its for the best). Or worse, she'll say its no big deal and then come to your wedding and make a big deal of it.

The only other thing I would mention is that if your friend went through the process of becoming a legal officiant already, I would reimburse them for any costs they've already paid.

Prior to our wedding, I was all for having friends marry couples because to me it always seemed weird to have some random person who doesn't know you at all be the person who leads this very private and personal event about you and your partner and unifies you in marriage. But there are definitely some advantages to hiring a pro, including their level of professionalism and not having to deal with weird dynamics if/when friendship issues or unforeseen circumstances pop up.

In our case, originally we asked one of my friends to marry us, a close male friend of mine who is reliable and trustworthy and whom my partner thought would be good at it. I was only going to have one bridesmaid, my husband was going to have four groomsmen, so having my friend marry us made things a bit more even. Well my friend wasn't eligible to become a JP in our state, but his wife was, so the plan was for her to be our legal officiant and sign the papers and for him to run the ceremony. When we had to cut the wedding down and have a 10 person elopement due to Covid, I sort of regretted going that route. We considered eloping just the two of us (which is what I wish we had done), but we didn't have any other officiant lined up and it felt weird to include my one friend and his wife and no one else, so we decided to do a small ceremony instead and max out the 10 person Covid limit and having a super small ceremony with just our parents and a few close friends. Having my friend's wife be one of the 10 people at our wedding just to sign our papers wouldn't have been my first choice, but we were stuck with that arrangement. To her credit, she managed to leverage some of her personal contacts and get sworn in as a JP when the courts weren't open at all, so I'm not sure anyone else would have been able to get officiated during the peak of the pandemic. Our elopement ended up being super stressful and disappointing, and even though I'm glad we got married, I wish we had done it differently. If we hadn't been relying on a friend to marry us and just hired a pro from the start, it would have been much easier for us to just un-invite all of our guests and get married on our own and do something really special and unique and completely centered around us and no one else.

Be honest as others said. Polite, but assertive.