You should have said no, or made her explain how to do it. I'm not sure why you'd use a high-temperature appliance on a loved one's head without knowing how to use it. Are you so afraid of your girlfriend to refuse such a dangerous request?

You should’ve just TOLD HER “Hey I don’t want to use something that can get to oven temperatures on your hair without knowing how to do so”. Failing that, you should’ve 100% told her that you fucked up so she could’ve either fixed her hair to hide it or cancelled her plans.

Ohhhh you fucked up… you fucked up biiigggg! Rip dude! Make sure you’re affairs are in order cos she about to rip you a new one! Haha

We don't know that OP didn't tell his girlfriend that he'd never used the straightener. She could have very well said there's nothing to it and given very basic/simple instructions. This is totally her fault, not OP's. If you need to do makeup and hair, plan accordingly so they don't have to be done at the same time.