If she has been diagnosed with ADHD, she has a health impairment that is covered by the ADA and therefore is qualified for accommodations under a 504 plan if her disability impairs her school life in any way - academically, behaviorally, etc. The school cannot refuse to accommodate her.health issue.
That said, a 504 plan won't help her magically do her homework. What a 504 plan will do is, for example, state that the homework assignments must be accurately made available to YOU in a timely manner (meaning via a teacher or school website, daily e-mails home to you, or teachers signing her assignment book to make sure that she wrote the assignments down accurately). Then that gives you the information that you need to more closely monitor her homework. Another part of the plan could be that if she is missing an assignment, the teacher is to notify you immediately and give her the chance to make up the missing assignment for partial credit (many teachers already do this, but it sets it in stone for those who don't). If her grades aren't on-line already (many middle and high schools post grades on-line in real time), a 504 could specify that you are to get more frequent reporting of her grades to help keep her on track before she slips too far behind.
Asking for help is something that all students should have access toso therefore that wouldn't be an accommodation - it would be your job to monitor her grades with her frequently or check her homework and then if she seems to not understand something, she'll have to ask for help or stay after.
My oldest (14, 9th grade) has ADHD and was on a 504 in grades 2 & 3 and has been on an IEP since 4th. He spends one periods a day in academic support. And still had a miserable first report card. We are once again back to my needing to check his assignment book, check that against what's on-line to make sure he has the assignments written correctly, check the on-line grading portal daily to make sure he handed everything in, helping him block off time for studying and writing, and having him literally sit at my kitchen table and do his homework or writing or studying in front of me. It's maddening but without constant supervision, he doesn't get things done, or does them halfway, or doesn't hand them in, doesn't ask for an extension or make-up. My boys in grades 1 & 3 take more ownership of this than he does. He's grounded right now until he has no more missing or late assignments and all test and quizzes are an 80 or better.
If I were you, if communication around homework and grades are an issue (you have no way of knowing whether or not her homework is complete, or that she failed a test) then a 504 would help remedy that situation. But a lot of her problem, I'm guessing, is her own behavior. Yes she has a health issue that makes school harder. Yes she has a problem that impairs her ability to organize herself. That means that she is going to have to work harder and longer. Hold HER accountable for her performance. I have a hard time with this sometimes, but at the end of the day, my son knows how much work it takes him to do well - he has to just sit down and do it. I'll help him get organized and stay on top of what needs to be done, but if he chooses to not do it, then it's on him and he loses his social life until he's back on track.