To be fair to your argument, everything I've read so far about working conditions at Foxconn would be considered close to slave labor in the U.S. ... so yes, there certainly is room for improvement there, and yes, Apple as a major customer could have some influence over the working conditions there if they wanted.
However, it's also worth considering that good mental health isn't likely to lead to suicide even in difficult working conditions; that depression, the number one cause of suicide [1], is a mental health problem whose treatment might or might not be within the realm of a company's ability to handle; and that even Disneyland in Paris has trouble with employee suicides. [2] Although, again, to be fair to you, that article leads right away by saying that the employees there have been complaining about poor working conditions.
Finally, there are cultural considerations. What are considered terrible work conditions by American standards might not be so horrible when viewed by Chinese standards, just as American work ethics are seen as 20th century by other countries.
[1]: http://www.suicide.org/depression-and-suicide.html
[2]: http://thedisneyblog.com/2011/02/04/fourth-disneyland-paris-...
