As a parent of a school avoiding autistic girl, I agree with much of this. We've found that the approach @Sirzy suggested often works - unless dd is in the midst of a shutdown and 'too far gone' to catch.
I think it depends on a few things though.
Motivation - was it the child's idea to participate, or did they 'agree', because it was actually instigated by the parent. Dd instigated wanting to participate in a gymnastics competition. I was reluctant because I know she suffers from performance anxiety. She went, having practiced infinitely at home, showed few signs of nerves and loved it. It was the same when she decided to take a speaking part in the school play. Contrast that with sports day, entirely different outcome (panic attack).
Team ethics: even if motivation is there, and it was a child-led decision to participate, if people are relying on you, I don't think it's right to opt out entirely. So on sports day, I told dd we'd stay to cheer on her classmates, she could sit with me witn no pressure to participate. She ended up having FOMO and managed to join in for a couple of events. The following year she managed the whole thing - and enjoyed it!
Resilience building (sorry) looks different for different children. Some need to know that they'll be listened to in order to feel safe, remove expectations, and put them back in control. This leads to confidence and a feeling of self control and autonomy - a really important life lesson.
Parenting is also knowing your child's limits, when to gently push and when to know 'now is not the time'. My dd missed Young Voices this year and I lost £100 because of it. She was motivated, it was her decision, but it wasn't the right time for her and pushing would have delayed her recovery. It broke my heart (for her, not the money), but there will be other opportunities.
Fwiw, I think the op's partner was wrong. Going anyway to support the other gymnasts was exactly what should have happened. As it stands, op's dd may well regret it (if it was truly her decision to participate).