It might be the case that the trimmer is 'fit for purpose'.
Did it say on the box or in the instruction booklet that it was intended for use on an up-to thickness of stem/branch and your friend is expecting it to cut thicker woody stems.?
I have a cordless trimmer and the chap next door to me has a mains operated one. They are as different as chalk and cheese and, whilst my neighbour's will cut thicker stems, I would be unable to make a case that mine is not fit for its intended purpose.
Ron.