I think you answered your own question when you said "I would want someone to tell me if my child was acting this way." I, like you, would be forever grateful for someone bringing something like this to my attention. Having said that, I'm sure your hesitation is because you know there are some strange parents out there who won't be as appreciative when confronted with this kind of news. Given this, you must be prepared for some bad feelings and possibly some backlash if that happens to be how they react. I personally would tell them regardless of the consequences. I mean, what's the worst that could happen? They get mad at you? They call you some names and use some choice words in your presence? I guess there are worse things, but I personally would risk it. I mean, what's the alternative? Keep it to yourself and just wait to see if their child attempts to hurt themselves instead of trying to prevent it? A reasonable parent will be able to see you are only trying to help. Any other response is one you can chalk up to irrationality and possibly stupidity.
Oh, and one more thing, I noticed some moms suggested getting a counselor involved to make it more anonymous. If you choose to do that, you may want to consider that some schools have strict policies about things being posted on websites and they might decide that child needs to go to an alternative school or may even report the info to CPS or something like that. It can become part of their permanent school record and could cause problems for them in the future. If this just happens to be a phase for this girl and she's only writing these things to be "cool" to this other guy, then this will get blown completely out of proportion and you could do more harm than good. I guess you have to ask yourself, would you want the parents to come straight to you or to the counselor and get the school involved? If given the choice between the two, I would want someone to come straight to me.
Good luck!