He is, just as much as Vettel. If you analyze the incident, you see Webber react on Vettel. At first Vettel wants to go for the outside. As soon as Vettel slowly steers to the right, Webber does too and Vettel goes for the inside. Nothing questionable so far.
Now stop the video and compare the available space left of Webber when Vettel goes for the inside to when he's fully alongside: Webber really squeezes out Vettel, it's not a matter of just being there. Since the difference in speed was that big, Webber should have returned back to the grippier outside line allowing him to counter if Vettel overshot the corner. That's what every intelligent racing driver would do, as it provides you the best opportunity to succesfully defend your position. Keep in mind the accident happened at the beginning of the braking zone, not on a full throttle straight.
Although technically Vettel may be wrong for steering into Webber, Webber is just as much to blame for unnecessarily creating a dangerous situation. If I was part of the Red Bull team, I'd even fully blame Webber.