I didn't acknowledge someone who helped with my thesis, is this considered plagiarism?
What you are describing is more like a technical form of ghostwriting.  It is currently a matter of some debate whether ghostwriting ought to be also classified as plagiarism.  Whether or not it is plagiarism, however, it is most certainly academically dishonest to use this person's work without acknowledgement.  This is not a minor matter.  

Moreover, if you did not disclose the help that you were getting to your supervisor and your committee, then it may be very serious indeed.  Knowing about external help that you were getting might affect their judgement of whether you have done sufficient work to qualify for a Ph.D.  In fact, if they did not know, and enough of your Ph.D. thesis was done by this tutor, it might even be appropriate for your degree to be retracted.