Look at the time the trade was submitted and the time the post was made. If he realized his mistake and posted, then I would say you would give him this ONE mulligan. But if he posted that later because other people started telling him how stupid he was, then that's his own fault. Depending on what site you use, most of them require that you confirm the trade several times before it is actually submitted, so I have a hard time believing the number of times 'accidents' occur.
Still, I've sometimes tried to muddle through setting a lineup or something in the middle of the week on my blackberry which is nearly impossible and I'm sometimes never quite sure what I did. But I always check before gametime on a real computer.
If this is the first problem you've had like this, I would be inclined to give him a do-over this one time...but make it clear to the entire league that this one incident is a last warning to EVERYONE. Double-check your trade proposals because you are not going to make a habit of reversing these trades. The "well, you did it for him" excuse is not going to fly in the future.