You can get that stuff to paint on their nails. Trouble is, some of them don't mind the taste, plus of course, as soon as they wash their hands, it has to be reapplied. But if she's keen to stop, she may be happy to reapply it each time anyway.
What about some incentives? Say take her ice skating (or whatever her little treat would be) each week that she stops, then if she stops for a month, take her to Alton Towers (or whatever bigger treat she'd like).
I guess that could become a bit expensive though. Do star systems still work with 8-year olds? Like they used to have in primary schools (don't know if they still do), but with a bigger award after a longer time.
Plus, of course, always encouragement - and perhaps some treats when goals are reached - from other family members is always a bonus.
Best of luck. It's a really tough one apparently. I've known several nail biters and they find it so hard. But I've also known several ex-nail biters, so it can be done.