I also think it's fine to change the code someone wrote. Just because someone wrote it, doesn't mean it's the right way to do it. I often find myself rewriting the code, it's the natural process of code evolution. It just feels that it should be more readable, efficient etc.
Although, if the change is essential or it requires more pair of eyes, I'll just make a PR(MR) and let the people review it.
I think a better process is to discuss with the other dev, talking through how you think it could be improved. They (or sometimes you!) can learn something that will lead to improved in the future.
It's a bit like that analogy about giving a man a fish vs a fishing rod.
Although, if the change is essential or it requires more pair of eyes, I'll just make a PR(MR) and let the people review it.