I am honored when someone trusts me with their manuscript. It is an honor to be asked to read or critique it. I always try to treat their work with the respect I would want others to show my work. I try to respond promptly and coherently (I may be deluding myself on that last one) because they trusted me.
Remember that when you beta read or critique. They trusted you, don't destroy that trust, because someday you'll need someone to read for you.