I'm working as a software developer at a startup in Istanbul/Turkey where there are only 2 full-time developers (including me) and 3 part-time staff (tester, developer, ux designer).
Yesterday I had an appointment with a dentist, and while I wasn't at the office, my fellow full-time developer changed the data format of a webservice from which I gather data for our iPad application. He changed it because the part-time developer and I are developing another version with Apple's new programming language (Swift).
An hour ago I realized the old version (which is still in use by our client in Germany) is crashing in a very ungraceful way due to last changes he made.
I tested the app last weekend and it was fine, so I definitely know it crashes because of his code. I suggested he make a copy of his latest work and simply undo his changes for me. After undo, he could add his latest work, so both versions will run properly.
The reaction I received was in a very loud sound, and he was saying (politely and shotly translated here) "I'm not doing it". If i simply said "shut up" and left the room, I might have to punch him in the face.
We were really good colleagues; I did nothing to upset him. He's also the cousin of the founder but it's not all good between them. I've had to cover up his big messes twice already. Just 2 weeks ago I was with my boss at 23:00 at the office because of his mess. Eventually I fixed the problem but it's now causing a delay on my deadlines.
Next month my masters degree courses begin in addition to a Microsoft certification course. I don't want to cover for him anymore and miss my deadlines, but sometimes parts he makes are used in my work. What should I do? I only wrote problem on Trello and tagged him along with freelance dudes, and take a screenshot of it.