My mum always said to me
" if you can't find anything nice to say, don't say anything at all"
She taught me that even if i didn't like someone i should still try to find a positive, like they had nice shoes or nice hair.
He was not asking for you to lie, in fact he wasn't asking for your opinion - it was a 'show and tell' post not a 'crush my spirit' thread.
There could have been 50 people look at this who had an opinion either way but chose to say nothing. And those that liked it, posted positive feedback.
Just be a little more considerate?