They should throw the whole thing out, since congress didn’t bother to sever the mandate from the rest of it.
But these days, who knows what the court will do?
Even had Congress included a severability clause in Obamacare, it would still be proper for SCOTUS to strike down the entire statute if it finds the individual mandate to be unconstitutional, since without the mandate Obamacare can’t possibly work, and it would be impossible for the Court to determine which of the ancillary sections (such as adding funds for healthcare for Indian tribes, etc.) would have been adopted even in the absence of Obamacare. (A severability clause, however, would have directed the Court not to strike down the entire statute had something ancillary, such as the Indian tribe provision I mentioned before, been found unconstitutional.) Let Congress determine which things it wants to keep from Obamacare’s carcass by having it pass those provisions as a separate law.
Since this wasn’t put into the law, if the court did it it would be legislating from the bench.