WA state legal age of consent in relationships

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A friend of mine has an 18 year old son who has recently started dating a 14 year old child, he claims that the law reads that its ok as long as the age gap between them is not more than 48 months. I cannot find anything confirms or refutes that. Any help would be nice, I think it's absolutely wrong, but I need something solid to show my friend to put this to a stop.