Wow! I think it's normal to feel some angst about 360 feedback, but you are in quite a predicament.
I'm debating on whether you should be being brutally honest in the 360 or sugarcoat it. I'm leaning toward the brutally honest approach. :) Whereas I think your consideration is commendable, I believe you need to look out for yourself. I’m outlining my advice in four points for you to consider:
If you don't give accurate feedback, you will continue to have a co-worker you can't rely on, and you have to do the harder tasks and projects, and is likely paid the same (or almost the same) as you. Are you content with this scenario? If not, an accurate 360 will document her performance, teamwork, and relationships. If you found out she was being paid the same as you, or close to it, would that change your mind? If anything, you're in the same pay range.
While you are doing all the difficult projects and working harder, have you considered how that might be limiting your performance? If you were only doing your job, you could do it faster and better and have time to take on additional assignments to help your career growth.
You're worried about hurting your co-worker's feelings because deep down inside you think she wants to do a good job, but just can't for whatever reason. In other words, the job is over her head, or beyond her capability. If she has ADHD, there is medication available that will help alleviate these symptoms significantly. It's not your fault that she hasn't sought medical attention. Have you ever considered that she fully aware of her shortcomings and knows she’s over her head? Your honest feedback may be helpful.
Your boss knows about your peer’s performance problem, but obviously isn't doing anything. Unfavorable 360 feedback may highlight her performance in such a way that action is taken. Perhaps she can move to a different role that is more in line with her capabilities. I know you can't make that happen, but it seems like it ought to be a consideration in the event it is ever mentioned.
Everyone gets negative, or at least developmental feedback on a 360. The feedback can be difficult to accept and hard to process. In the past, I facilitated 360 workshops to help people interpret their results, develop a plan and in the process I talked many people off the proverbial ledge.
Bottom line? Be honest but kind with your feedback. I hope this helps. Good luck!