Respectfully disagree. I don’t think there are any legal impediments to having NVS—or another company other than ABBV—pursue buyout interest in ENTA.
Though I agree with your statement, I think there is a very difficult issue for any non-ABBY suitor. Namely the value to place on ENTA's 2nd gen PI they have partnered with ABBV. This could well depend not only on the drug's data, but also on ABBV's market plans.
I would be delighted for a buyout (well, assuming it is after 8/31/14), but I do not expect such.