["I recently discovered that a coworker's decision have led to me having a big increase in my workload - it was super disappointing!", 'I am very sorry! That is very frustrating! Can you speak with a supervisor about the situation?', 'Yes, it was frustrating.  Our supervisor is pretty uninvolved, but I might need to speak with him to prevent this from happening again.']
I don't blame you, I would talk to him. Even if he is uninvolved, it is not fair for you to bear the full weight of the tasks at hand. I sure hope he has a solution and the work load eases!
Recently I had a meeting at work where I discovered that someone else's decisions caused me to have a heavier work load.  It was very disappointing.