Update to my last rant:

I wrote a reply to the person. Not scathing (as I'd have liked it to be) but firm and in a no nonsense way. My manager supported me. My project manager talked with the person to in order to convey what the issue is and to undo any misunderstandings due to written communication (we have different native languages).

I have not received a reply but my project manager told me that they are analyzing the problem now. I was also told that they are not a bad person. ^^
I think I'd like to believe that. We all make mistakes after all.