This is one of the main reasons that I dislike rolling for stats (I assume this is what happened). The easiest way to avoid this sort of problem in the future is to do a point buy system which will give everyone's character an equal opportunity to be powerful. This also provides a great deal of choice in the matter which is always great to have while making a character. As a player, the worst thing ever is to get crappy rolls that first day, and be stuck with them forever while your buddies were lucky that first day and benefit forever.
Retcon - The easiest is just to talk with the players, figure out the stat differences with the highest and lowest characters, and just give the lowest characters points they can spend to 'bring their characters up to speed' as it were. Avoid nerfing powerful characters, as this brings about resentment.
Items - Next, you could throw out some items that would be useful for that character. Imagine a belt of dex +4 for the monk, and what that would do for survivability. Or even monk's robes (expensive). The downside to this is that eventually, the fighter will get items like this, bringing back the problem again. You might even consider the various tomes that increase stats, but those can be expensive as well, plus your good characters will want those as well.
In-Game Events - Imagine the group walks into a temple and finds some altar of power. Those who touch the altar get a certain permanent benefit. You would probably take this opportunity to give the less powerful characters more powerful benefits (although you should give everyone something, else you get player resentment).