Well, he wasn't vigilant enough now was he? A collision happened. And I said, and still do say that his actions were casual even seemed lackadaisical.
At the risk of pointing this out again, I grew up in SoCal, and have had plenty of close calls. ATC says '12 oclock 2 miles' and I'm doing something, either turn, climb, dive. And I'm also scanning like heck(same as the F16) for traffic, but I'm sure not staying on that heading in VMC. Next, ATC asks if he has the traffic, and when ATC says 'immediately' I'd be on knife edge(or close too) before he was done talking, yank hard.
While 'immediately' may not be in the P/C dictionary(I think it is), it's common usage, and intent is clear in this context of the third call on same traffic.
If we postulate that 240Kts, then 4NM per minute, and 2NM in 30 sec, adding the latency of the scope sweep, he should have been made immediate correction at the first call of '12 oclock 2 miles'.