To me, that reads as passive aggressive. "It's unfortunate you didn't do your job..."
I think it's a better approach to say "The problems are X. We know this because of Y. My proposed solutions are Z. If you agree, let's get together and figure out how to do Z. If you don't agree, let's get together and figure out where our analyses diverge and how we can come together."
It is a fine line, yes, but if you abstain from any accusations in the rest of the text, then my formulation is generally received as empathy. This is supported by the fact, that there are no lies there. OP _is_ sorry that things went as they went. And it also is objectively “unfortunate” in the sense that with a little luck there might have been a different situation.
The general point is that OPs partner is in a corner. Empathy is needed to allow him to be cooperative and understanding of his own mistakes.
I realize now that at some point in the last few years I totally lost any sense of empathy for my co-founder. As I write this I realized that today was the first time I've shown him any real sense of empathy in a really long time. Coincidentally I think we may have actually made some progress in turning things around today as well.
I think it's a better approach to say "The problems are X. We know this because of Y. My proposed solutions are Z. If you agree, let's get together and figure out how to do Z. If you don't agree, let's get together and figure out where our analyses diverge and how we can come together."