Is it okay to edit another user's closed question in the hopes of getting it opened, or should that issue rest solely on the OP himself?
A lot of this comes down to why the question was closed in the first place.
If the question was closed for being unclear, but you can decipher it, rewrite it in clearer language, and make it useful, go for it.
If the question was closed for being off-topic, subjective, or too broad, odds are that any changes that would make the question a useful on topic question also represent a radical change from the question as asked. Thus, I'd generally discourage such edits; if you think that there's a good question at the kernel of what the OP has asked and they don't or won't edit their question to fit what you are suddenly curious about, ask the good question yourself.
As always, situations like this are fundamentally judgment calls. My judgment tends to fall on the side of encouraging edits that sharpen, refine, and clarify the meaning as presented in the question asked, and against those edits which seek to turn the question into a different, if related inquiry that more readily conforms to the rules.