I believe this is to do with differing cases and the "E" ending. I believe as a rule of thumb that most/all nouns ending in "E" (and a few other non-standard endings), that are masculine, are modified in the latter 3 cases by adding an "n" or "en" to the end. It's Der Junge for nominative, or Die Jungen/Jungs for "boyS".... but De[n|m|s] JungeN for other 3 cases. Someone else please chime in to correct me if I'm incorrect.
Also, the sentence can't read "She likes every boys." By context you infer that the sentence translates to "She likes every boy," where 'boy' masks a plural-case being preceded by 'every'. I bet another accepted translation would be, "She likes each of the boys" (in which case 'Jungen' is expressed explicitly).
Nope, because Jungen is singular in this case and it's in the accusative case. Some nouns in German add an n or en at the end when in accusative/dative/genitive cases.
Also, if you read the other discussion in this discussion, they already point this out.
thats what i thought but after reading comments i get it. there are some masculine nouns in german which ends at "e" like "junge, kunde, neffe, lowe etc" and these all uses "n" at the end in all cases except Nominative. there are some other kind of nouns also which ends at "n" or "en". you can see them in links given in comments