Tell her, whatever she was doing for year 11, she would need to stop doing that since its clearly not working out for her. Wanting to do good is different than needing to do good so ask her to have a goal in mind, for example her dream degree or career, make sure she can internally justify working extremely hard for something; you're not going to study hours on end for days just because you want "marks" (well for most people, I'm sure there's people solely motivated by marks)
Show her this forum perhaps so she can communicate with other individuals doing their HSC and ask problems if she has some. The community is small and tight-knitted so it doesn't suffer the same issues as most HSC forums or study groups and everyone's really nice and chill (for the most part, but you can avoid the 1 or 2 people who are a bit mean). I think it will help her as she can get help from a lot of people such as the mods like CM_Tutor for maths etc and there's a lot of resources here as well so its just not discussions, they're really helpful such as the physics and eco notes and there's plenty of threads that answer questions that you might have in the past since the site's been here for quite a while. Just seeing different solutions and ways for doing problems, as well as making mistakes in attempting questions for other people has improved my skill noticeably. There's a lot of people willing to help here and thats the great thing about this site.
Last thing would be is to slowly develop a study routine, don't just get into 6 hours bursts per day because at most times it doesn't help, you'll feel worse studying weeks and months for a subject and getting an average mark simply because you aimlessly studied, Quality > Quantity anytime. Tell her to study in small portions per day, starting out 30 mins per day and slowly building from there - ensure there's a reward for doing so, perhaps extra TV time or some snacks lol. This helped me a lot in reducing my bad habits such as procrastination and this is a broad topic because some techniques and strategies, such as the one I mentioned, don't always help as everyone's different - so some people might not like pomodoro technique while others would highly suggest it. Personally I don't do these strategies, I've gotten to the point where I can just focus on my work for a good 2 hours then go take an hour break.
You can't really give her life-changing advice, its really up to her and the extent she's willing to go for the ATAR. A lot of people want to be X but they're not willing to go through the journey to get to X. You just have to make the decision and just commit - there's no shorter route. You would know your friend better than anyone here so you could help her the most, perhaps studying together can transfer your habits to her (if you have a good routine yourself). All the best to her and you.
