What Am I Supposed To Do?

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Well based on what you're saying I'm assuming your friend's crush is pretty good-looking then?
Honestly if she's pissed that you gave her crush a compliment then she's not really worth being friends with.
Sometimes, just because your friend has a crush on somebody it doesn't mean that you have to go out of your way to make her happy.
If I were you I'd just let things to die down and eventually she'd let it go.

Maybe she's pissed right now but well the rage eventually dies down anyway since this isn't even that big of a deal to begin with.
If you don't want her to be mad at you then just ask her if she thinks it's alright that you posted that on her crush's timeline! (without telling her that you knew she was pissed)
She can't be that pissed at you for asking her (even though you did post it already, but well there's nothing else you can do now)
It's alright really things like this happen all the time and sometimes friends get bitter and jealous at the slightest things when it comes down to crushes and competitions.
Just know that she'd forget about it eventually and would stop being pissed.
Hope I helped!