Unless you have some sort of guarantee that Rob was going to deliver on his tax returns, I think you'd be foolish to put up $350 as security for Rob. Rob's a nice guy and all that, and I am sure you have respect for him, but $350 is real money. It's not $5 for tipping a cocktail waitress at the video poker machines.
To be clear: If you would put up the $350 as security, and Arc sends in his returns (for $350) and Rob doesn't follow, you are willing to forfeit the $350 to Arc as compensation?
I think the alternative (and a wiser choice) is for both parties to have their TRANSCRIPTS sent to me with NO out of pocket costs. Let's at least LOOK at the transcripts. If they don't break down gambling wins and losses as I suspect they will, and Arc suspects they will, then we'll have to look for the full tax returns.
In short, why spend money and waste time for full tax returns to be sent through the mail when TRANSCRIPTS can be sent to me in a matter of days????