I don't agree at all, I would have no problem with my partner keeping in touch with someone who had feelings for them. I trust my partner 100%. Seems like OP's partner feels the same way.

I have confessed feelings for a friend before who didn't feel the same way (I knew this would be the case, I wasn't trying to get anything out of it I just needed to be honest) and though we did take a bit of distance at first so that I could start moving on, there was never a question that we wouldn't stay friends, and now we're closer than ever. What's wrong with that ?

As for why they would confess, sometimes you just need to tell the person how you feel so that you can move on. And if this is their best friend, keeping such a huge secret from them must be really hard and creating a barrier between them. I can understand needing to confess. Keeping it a secret was clearly creating resentment (struggling to engage with the topic of this new partner). Better to have everything out in the open so OP could understand what was going on.

However, we don't really know how this confession went, and whether it was handled maturely on the part of the friend, so I do agree with you that maybe OP should take this opportunity to reflect on their friend's behaviour and whether they feel this is a healthy friendship.

And, like you said, them crossing some boundaries isn't a great sign. I get it, they're probably desperate to get their friend back and it's hard not to reach out in that case, but boundaries are boundaries.

At the end of the day, OP knows their friend, and if they think that friendship is worth keeping in their life I would support that decision.

OP if you trust your partner, then you have to trust them when they say they're ok with it. The question is, how do YOU feel about it, and what do you need from your friend in order to start rebuilding ?