I hope Boeing have given proper thought to just giving up on this contract. It's a huge money pit for them and they'll never make it back.
Thing is, without a human-rated capsule they are locked out of any future Crew contracts.
NASA aren't going to hand out any more contracts to R&D a capsule - they already did that and Dragon works fine. Boeing have had a very generous chunk of cash to develop Starliner and their options are to make it work, or walk away ceding whatever glimmer of credibility they have left and making themselves the also-ran in future contests (because they're now a high-risk supplier).
So if Boeing exit this programme, they're basically saying "We're exiting the manned space market and are incapable of competing with SpaceX".
Now, we already know the latter part of that to be true. Nobody can compete with SpaceX at the moment. They're leading the market on price, availability and reliability. But NASA would very much like to have Starliner there as a backup to Dragon. Without getting into sunk-cost fallacy, Boeing probably are far enough along now that it makes no sense to cancel. They're got Starliner to the ISS and back in one piece (unmanned). It's basically done.
So there's two things :
1. Unless Boeing wish to abandon that market sector entirely, they need Starliner (or another human-rated capsule) to offer when bidding on crew and cargo contracts.
2. Starliner is basically done - albeit wildly overpriced. They'll really want to get their contracted launches done and then be able to bid on new contracts to make some money back.