Good News: There's no real risk to them remaining on ISS at this point, short of some major incident on the ISS itself which required immediate evacuation.

Less good:

Even if Boeing had a spare Starliner (they don't), they don't have a spare rocket to put it on.

Even if Boeing had a spare fully stacked Starliner + Rocket, I'm not sure anyone (even Boeing) would trust their process to be agile enough to launch a recovery mission any time soon.

So that leaves Russia's Soyuz and SpaceX's Crew Dragon.

Using Soyuz would be a major political headache, aside from anything else. It'd require scrapping the next planned Soyuz mission - it may take several months to get that ready. I believe they have to (and would) have to send it up manned.

Using Crew Dragon may be possible, No idea on where SpaceX is as with having refurbished/ready to go capsules on hand, but they have enough launch cadence that it may be possible.

Space Suits then become a problem - they're specific to each launch vehicle, and customised for each wearer. So lead times there become an issue.

Though that would leave the current crew stranded up there until a replacement spacecraft can be sent. That takes several months.

This has happened before. The Soyuz docked to the ISS in Dec 2022 was damaged by a micrometeorite strike and was deemed unsafe, leaving that crew essentially stuck up there until a new Soyuz was sent back up in Feb 2023. (The damaged Soyuz was later remotely flown back empty.)