Interesting question. But I would say yes, your friend could be sued.
In reality what the TOU is saying is that Blizzard is not bound to recognize the "account" as an item of property (like a car or sofa). Blizzard owns the account and content, not the end user.
The distinction is between "ownership" of the account, and the "right to use" the account. What your friend sold was the "right to use" the account. That is why outfits like vbarrak still exist and haven't been sued out of existence by Blizzard. Basically, the TOU means that neither your friend or vbarrak can actually force Blizzard to give them the account, and why vbarrak cannot sue Blizzard to get the account back that your friend sold.
In any event, one issue (Blizzard's EULA and TOU) has nothing to do with the other (Friend's sale of account access). Your friend sold "something" to vbarrak and vbarrak paid your friend money; your friend took that something back; vbarrak has a claim against your friend. Vbarrak can sue over it. Whether they actually will is a different issue, I doubt it they will.