Remove it. Sand flat and reapply the tape correctly.

Apply a thin coat of drywall compound(mud). Embed the tape in the mud with the drywall knife. Add a light coat of mud over the tape. Let dry 24 hours(unless quick drying(hot) mud is used---which is not usually DIY friendly.

Sand lightly and add another coat---with a wider knife to feather the application. Let dry and repeat with a still wider knife.

Sand very lightly and prime with a drywall sealer.

Thanks. We'll see if he pulls it off or tries to go over it. They mix their own mud.

Would this have happened anyway even if the next coat/s had been put on? Or would they have locked it down?

If the tape is not applied over a sufficient coat of mud, it will never stick and will always bubble or lift.