You don't need anyone's permission to mention a fraternity in a fictional context, especially in passing! (I'm sure it would be different if you wanted to write a whole book.) Usually when something like this happens it's because someone involved is in that org.
Obviously, if what had happened was extremely negative ("Remember the time we hazed the pledges?") it would upset the fraternity, but usually these are value-neutral or positive references. It's good PR!
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Alpha Xi Delta
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