You'd be very unwise (and dishonest) to make something up.
The decision to involve you in the surgery was made by the surgeon - assuming you didn't mislead them about who you were, it's fundamentally their responsibility and if the decision was inappropriate it's more their fault than yours. If asked about this experience, make it clear that you realise that there are potential problems in involving untrained people in theatre (include a mention of full patient consent) and that medical students should know their limits, and not operate outside their comfort zone in a way that could endanger patients.
You were inexperienced and when asked by someone superior and knowledgeable to help, you did as you were told. I don't think anyone would hold this against you if you're clear on the problems and what you've learned.