You are right. No one knows what the person on the other end is going through and I would think that if they have to rehome a pet, it really is a last resort. I would assume that they are making that decision because they love the pet so much that they truly do want what's best for it.
Generally speaking though, it really angers me to see people rehoming a pet because it 'got too big' or to rehome one for whatever reason and then turn around and get another, 'newer, better, pet.'
Sadly, that is the case sometimes and it just isn't right by the dog(s) involved.
I am sorry that your friend got some rude comments and I hope that he/she can look past it and is able to find the perfect home for her little one. Best of luck to her/him.