Citing a result due to a single author that appears in a paper with multiple authors
I would just cite it as "Jones and Smith" and not worry about it.  The standard in math is to cite papers by their authors.  If Smith wanted to be cited alone, she should have published the result herself.

I think this situation has some precedent in other fields.  I might be wrong, but think some journals such as Nature (see http://www.nature.com/nature/authors/gta/#a5.5 "author contributions") make the authors disclose who did what.  It does not mean that the paper needs to be cited differently depending on what part of it is used.