Not really. He has his screen on for a little while, but then it's hardly on at all and the drain is more. 3G usually takes more than WiFi. (As stated, as long as you have an actual WiFi connection.
The screen on time does have gaps in it during the drop, but it's still on a lot more than it was on either during the minimal drain during wifi on or during the minimal drain when wifi was off. The phone was also nearly always awake during the large drop (some of this is probably due to data being on and downloading/syncing or whatever, but data alone won't, or atleast shouldn't, cause that drop).
So IM(admittedly noob)O the drain is due to him actually using the phone (a small amount will be because of data, but it won't be a lot), rather than it just sitting idle with the screen off. The period of minimal drain afterwards should show that just enabling data alone isn't draining the battery.
Although it would have been easier to tell what exactly the cause of the drop was if he'd screenshotted the battery stats as well as the graph.
Edit:
Actually I've looked at the graph better now, and I think we're talking about different things

.
I didn't realise that the battery after the large drop is still dropping slightly more than with wifi on

.
I had just looked at the large drop because he said that the battery plummeted, so didn't look at the part that looks like fairly normal drain

.
Apologies if this is why we're disagreeing, as I'd agree that that battery drain after the large drop could be because of data

.
Oh, and I'm on my phone, so I blame the small screen for the confusion

.