I agree Ian's reaction was completely credible but, it was a big risk for Julian to take. He can't have known that Ian and Adam hadn't resolved things and Ian seen through Rob entirely. Which on the whole he has and always did, except that he hasn't worked through how much of what Rob told him about Adam was fact and how much conjecture.
Ian could - should? - have been prepared by Anna to have any dislike of Rob, on his own account, brought up.
But, what interested me was that Julian appeared to go against his own better judgement to placate Rob. He said he wasn't going to bring up Rs statement on Ian, he almost didn't, paused, then, after the judge prompted him to question Isn or move on, he did.
I'm sure that really he was evaluating exactly how to phrase his questions but, it sounded like a 'what the hell, let's see where doing what R wants takes us'.