My husband vomited soon after we got out on the sidewalk once. We never knew why.
If it is going to happen or happens without warning, you would take off her BiPAP mask, of course, turn off the machine, and support her abdomen so as to help her fully expel her stomach contents, as well as to cough and/or spit to whatever extent she can. You would clean out her mouth to avoid her reswallowing anything, and try whatever she thinks would help reduce any residual nausea.
Suction might or might not be helpful, but once she is no longer nauseated, if she was using the BiPAP, going back on it can help, though possibly at a lower pressure and slower rate for a while.
I would try to retrace what the cause might have been for her nausea today, but sometimes it's unknowable like anyone else.
Best,
Laurie