Allowed help in thesis
From a publication point of view:

In general, this is nothing else than peer review and totally fine.

As long as the other person only tells you "all good", or "this here doesn't look good, maybe rewrite it so that X is more clear", that is no problem at all. If the other person writes the whole new version for you, that is obviously not good. Either way, this wouldn't be plagiarism in this case. Plagiarism means copying from a different paper/book/source without properly mentioning it. What this would be is someone else writing (part of) your publication without getting mentioned. That is at least as bad as plagiarism, but it is something else.
Depending on the level of help you got, consider mentioning the other person in the acknowledgements. 

From a university point of view:

A thesis isn't completely the same as a paper, so here, other rules might apply. At some universities, you have to sign that you didn't use any sources or help apart from the ones clearly mentioned and cited. Thus, rules might be stricter here. While I would not worry about, say, a math major getting feedback on their English grammar by someone who has no idea about math, as soon as the one reviewing your work is from the same field and/or also gives feedback regarding content, this is something that should be cleared with your advisor first.