Credit with helping to write a PhD thesis I am an undergraduate student doing summer research and I am helping a PhD student to write a portion of her PhD thesis and with a bit of editing here and there. I am wondering how would credit be distributed since I don't think a  PhD thesis can be co-authored. Also, when I apply to graduate school afterwards, how would I show the admission committee that I have done this work? 

To clarify: I had two kinds of contributions to this thesis. There are parts describing research we collaborated on, but it was mainly me in that I collected the data and performed the analysis but she gave me advice. There are also parts that describe her research, that I helped to edit.
In some fields, almost all work is done as a collaboration. In these fields, it is not at all unusual for this collaborative work to end up in a student's thesis. However, the thesis text should clearly state "This chapter describes joint work with X" (e.g. in a footnote).  

However, to really get credit for this joint research, you should prepare a manuscript and try to get it published, or at least release a preprint you can put online (if there isn't time to get it published before you apply to grad school).

For the editing assistance, the most you can get is a thanks in the acknowledgements, and help with editing is not likely to impress an admissions committee.