Valeant, no; it has no standing to dictate anything to Allergan -- making a bid for someone doesn't give you a right to tell them how to run their business and have the courts enforce it.
PS, is another matter -- it's a major shareholder. Ackman stated that if Allergan tries to make a major acquisition he'll sue. However, this was before he was himself sued (and SEC-investigated) for Insider Trading, where his defense is that he is a co-bidder. If he is a co-bidder, he is in the same boat as Valeant. Maybe he can find some creative legal gymnastics that somehow makes him both (we are in uncharted legal territory, as I do not tire of saying), but the courts would want to clear that before giving him any rights to interfere with Allergan's business.
While a judge can block a defensive transaction, it's a high barrier. Unless proven to the contrary, Boards operating within the Bylaws are presumed to act in the best interest of shareholders, and a judge would loath to substitute their own judgement for that of the Board. Whether Ackman can prove to the DE courts that this Board is no longer acting in the interest of the shareholders remains to be seen. So far, twice, he was able to convince the court that there was need for expediting; but there has yet to be any rulings on the merits of any issue.
Dan.