In contrast to "sad songs" there are "happy songs". The original has frenchified "happy songs" into "happies songs" applying the French rules of
liaison http://french.about.com/library/pronunciation/bl-liaisons.ht... These are rules of pronunciation and should not be applied to writing. Maybe the writer had "blueS" in mind". No excuse though as "blues" is plural in English.
If the French is being published, you might gain a brownie point with your client in pointing out their error. Unless they are overly sensitive, in which case you might get howled out of court! Sometimes foreign words are changed in another language. It's not the case with "happy songs" though. It's just a mistake.