You need to check your university's guidelines on this - some will let you use an editor, others won't. I wouldn't suggest an editor, as these can be very expensive, and can lead to a lot more work if they want you to change sentence structure/arguments etc. But I definitely recommend getting a proof reader - and a professional one, not just a friend. I have someone proofing my thesis, and the things she's picking up are amazing - lots and lots of small mistakes, and inconsistencies. I've checked my thesis and just didn't see these mistakes. These are the kind of things which annoy examiners, so I highly recommend getting a proof reader.