The reality is the relationship breakdown had started long ago. As much as Ramirez once looked up to Harry, once he worked with him and developed a relationship with him he became painfully aware of Harry's flaws and failings. As they say, never meet your heroes.
The catalyst that broke the friendship might have been the deaths of Yoshimo and Wild Bill (and possibly Chandler). But really it was already fairly damaged well before then. Trust had been lost on both sides for a long time, they had been moving in different directions for some time now, and it's unsurprising really they are on a collision course.
Like all relationship issues, it never comes out of nowhere and it's never just one thing (even if one or both parties might think that). It's always a series of things, there are always things that have been building up.
I don't know that Harry "should" have trusted Ramirez during the events of Peace Talks (and of course Battle Ground) considering the relationship breakdown by this point. Work would have needed to be done much earlier to avert the events that followed and subsequent collapse of trust. But perhaps Harry should have tried to mend the relationship and put more effort in years ago, certainly prior to the events of Changes. After that...there wasn't much time for Dresden to work on any relationships. Then again, it's hard to say how much effort Ramirez put in to earn that trust either. We only see Harry's perspective and that tends to skew things.
Tactically, it might have been wiser to let Ramirez on a few things (especially with the benefit of hindsight), but that being said Harry clearly saw Ramirez was working against him already by this point. It's like giving a cop the information to sabotage your efforts in trying to do some good, even if it isn't how they like it, and then they arrest you or whatever. Harry made probably the best choice he could with the available information at the time. Doesn't mean it was the ultimate best choice but Harry is neither omniscient or infallible, hardly even close.
Don't forget, Ramirez could have chosen to trust Harry some more too. There are consequences to that as well.
