I don't think you should put her down.
The severity of the bite to your friend's son isn't really very clear, but it sounds like your dog isn't untrainably vicious or something along those lines. It sounds like she's a good dog that's just not good with young children. If she's amazing as you've stated, then she shouldn't be put to death because of a mistake in judgment or supervision.
As pet owners, we claim responsibility for their care and their actions. It wasn't mentioned if training (professionally or personally) has been tried, but that's always an option. If for some reason that isn't possible, then she should be supervised or kept separately from children. If you find yourself unable to provide the best possible care for her, then you should be looking at no-kill animal shelters or rescue agencies instead of on Craigslist. There may be a fee to give her up, but pets are always a commitment.
I have two dogs that I've rescued, one which is very bad with other dogs. It's been four years and he's only just started listening a little. If I want to take him to the park, I have to do so at an odd hour that isn't peak time. Sometimes I take him to pet conventions so he can have some interaction, and I have to prepare myself for embarrassment or bites. Some people are cruel, and it's best to ignore them.