I was wondering the same thing when shopping for my Liver Flush ingredients. I read all posts on curezone that had "malic acid" in them and I found only few people advising against it. I can't find these posts I initially read but these may be helpful links:
//www.curezone.org/forums/m.asp?f=263&i=18

http://askshelley.com/forum/post-994.html&highlight=#994 OR the same at
http://www.askshelley.com/forum/about268.html

However, some people apparently use it:
//www.curezone.org/forums/m.asp?f=447&i=18763

Malic acid is cheap and can be easily ordered online. I wouldn't take any chances with this. The flush may work in either case.
You may want to read more about malic acid on curezone and maybe you'll find a more satisfying explanation.

Regards,
Westley