I suppose we'd add the "s," but my best advice is not to use it at all, because it's redundant, if not absurd. In order for this to make sense, she would herself need to be one of her friends, if you're speaking of three peoplel If prior context mentioned yet others of her friends, then we'd be talking about four or more people. I'd suggest, She is moving [in} with two of her friends.
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