I’m currently reading the second doctoral dissertation within a couple months. Both are plagued by the kind of language errors and typos that you really would not expect to see in these kinds of literary products. How long does one work on a typical doctoral dissertation anyway? Years, in any case. So is it too much to ask to proofread your work, huh?! Not only is badly written text highly annoying to read, it very easily destroys the credibility of what might otherwise be a fine dissertation.
As you know already, the most important thing is the content of what they are saying. Typos are human error, but sure in an ideal world, typos wouldn’t exist. Imn even sure the student has realised this after submission and though “oh crap”. Dont let this affect your marking, besides publications almost always go through final editing before hand, as will this eventually.