Preparing to support
If someone is preparing for an experience and you may be the one who stays, a few plain things are worth settling in the days before — for them, and for you.
You may want to consider: what has been agreed about what you are and are not present for; who else could be called if you need a break; and what your own limits are. Supporting someone does not require you to share the experience, or to have a strong feeling about it.
Your limits are part of the plan, not a failure in it. Settled ahead of time, nobody has to negotiate them mid-event.