What is a reasonable rate to charge for editing someone else's thesis?
I don't have a better estimate than http://www.the-efa.org/res/rates.php whose rates work out to about 2 cents a word for basic copy editing or about 40 dollars an hour. For a thesis which is going to be very technical, I would expect someone with relevant expertise to get more. Grammar and form, probably is close to basic copy editing, but is probably more involved than that.