My boss is leaving Saturday, March 3 for a 12 day trip to China. Does anyone know if she can purchase Diet pepsi and M&M's in China or if you can bring them into the country? Thank you

Anyone who can't live without Diet Pepsi and M&Ms for 12 days in China shouldn't bother getting on the plane.
It depends on where your boss is headed in China. I don't recall seeing much chocolate in China.
I have seen M&M's throughout China (peanut and regular M&M's can be found even in Tibet).
Not too sure about diet Pepsi. I don't really remember seeing it, though it might be everywhere and I just haven't noticed it. Seems like I have seen diet Coke more often than diet Pepsi.
Yes, you can get both M&Ms and diet Pepsi. There's more diet Coke though if your boss can lower to that standard.

but tell her not to be tempted to try local Chinese chocolate... a warning: theres a reason why its cheap

I would wholly agree with #1.
Where is your boss going in China? M&Ms widely available but diet Pepsi/Coke isn't really common at all.
Cheers!

I wholly disagree with #1.
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<hr>I don't recall seeing much chocolate in China. <hr></blockquote> #1, have you actually been in China? There's bad Chinese chocolate everywhere, but follow #5's warning and avoid it if you don't like the taste of wax. Grocery stores and the occasional mom and pop store have Dove chocolate bars and M&Ms. Large grocery stores have more choices.
A business traveler shouldn't be faulted for wanting comfort foods. Actually, I don't think anyone should be.