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US immigration does usually let family members join US citizens in the citizen line. This does not extend to friends. As always, there's a possibility you'll get away with it, but if not, the non-US citizen will be sent to the back of the other line. Source: I'm a UK citizen with US citizen family.
Depending on the terminal you enter, whether your friend has an ESTA and has visited the US before, they may be eligible to use the kiosks that allow quicker immigration now.