Can I stop helping backstage long enough to watch the play?

I bought a ticket for Friday, then the costume coordinator said, 'We'll need you behind the curtain for the quick changes. You can catch the story from there.'

A friend is in an amateur production. I offered to mend two jackets after a rehearsal and stayed to sort the costume rail. That turned into helping at three more rehearsals. I enjoyed being useful, but nobody asked me to join the crew for the run.

The coordinator has now put my name against every performance. She says training someone else will take time they don't have. My friend says she'll feel better knowing I'm there, which is lovely until I realise she means backstage, not in the audience.

I can help on Saturday, but I want to sit in the seat I bought on Friday. How do I make that clear without leaving the actual crew to discover the gap just before the first costume change?