We’ve said it once before in this list, and we’ll say it again: friends’ ex-partners are off limits! This rule applies to anyone your friend has dated, casually hooked up with, or even flirted with (in some cases). Girl code explicitly states that you should always side with your friends over some guy or girl they used to date.
Even if your friend simply has a crush on someone, that marks them as “off limits” because it’s courteous and respectful to your friend and her feelings. Don’t insert yourself and start a competition between you and your friend for a romantic interest’s attention. That’s extremely petty and will start unnecessary drama and heartbreak.