Using "les" would be an instance of "leĆsmo", an anomalous construction, in this case, a somewhat accepted use -not in my country and a few others, where it sounds something between horrible and excessively polite-. Using "los" is the only correct way -for me, it is; for others, they'll have to reluctantly say it is though they may keep using "les"-.
About "los", you should use it whether they are 10 male friends or 1 man and 9 women as "los" is in fact the default for genderless, something most native speakers don't get.
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