"She comes afterward" should also be accepted. Reported. http://oald8.oxfordlearnersdictionaries.com/dictionary/afterward#afterwards__8
No, because is coming adds the auxiliary verb ("to be" = is in this case) and turns the descriptive verb into an present participle (-ing) which takes this exercise sentence from Simple Present Tense to Present Continuous Tense; but that is beyond the level for which this lesson originates on DL (Adverbs which is concurrent with Present 2 Verbs):
But, "She is coming..." would be, "Ela está vindo..." with the LOCO (locations/conditions) temporary version of, "to be" estar used in PT. At best we can get, "Ela vem mais tarde"