Pretty sure there is no issue sharing your ST with friends - the club has said it will turn a blind eye, has it not? Sure there are loads of people who have shared STs anyway.
We have this. I sold mine for the Manure game. Worked like a dream.
Think it is even in Man Utd's terms that you can pass your ticket to family and/or friends (or at least it was in the terms for 2017/18)