I don't think Stef gave Brad the wrong advice and I think Brad analyzed it out right, but I think Robbie sleeping with other people was inevitable, because he really wanted to. And I think there is someone specific he wants to do. I don't think it is 'just a twink in the office', but someone who is fueling Robbie's lust and maybe fanning the flame with actions and nuances. Hollywood/West Hollywood is full of shitty little twinks who'd happily break up a marriage to get a Mercedes Benz, a nice house and money to burn.
If Robbie really wanted to stay faithful to Brad he'd be asking David to help him with coping mechanisms so he could stay faithful.
As to business, if I was Brad I'd write down the name of every guy at Gunn I'd pissed off, Dan, Doug, Lark...... well it is a long list, and then hire an investigation firm to do a work-up on each individual.