Guilty as charged. This is not even remotely my practice area so it is more my gut feeling from having read a lot of legal pleadings, but based solely on what is stated in the complaint as currently drafted, my opinion is it is not a very strong case.
There is probably enough to survive summary judgment (i.e. a determination by the court that there is a disputed issue of material fact on which relief can be granted if the pleading is cleaned up) with respect to Isaac, but that just means the case can proceed to trial. I think it will be very hard to show that it is more likely than not Isaac caused it. Nobody shows all his/her cards in an initial pleading, so they may have more persuasive evidence, but if the texts and a statement she abstained from relations with others before becoming symptomatic are all they have, it is not a very strong case.
With respect to Purdue, I would be surprised if the claim survives to trial. The complaint asserts tort liability with respect to Purdue. In order to state a tort claim, they have to show Purdue owed the plaintiff a duty of responsibility. As I said not my area of expertise, but I would be shocked if there is case law holding that a medical clinic owes a duty of care to a third party who may in the future have intimate relations with a patient of such facility. If doctors' had such a duty, they would have to post public notices saying John Doe/Jane Doe has disease x which is contagious in a manner reasonably calculated to provide notice to avoid liability.
