Only after her mother came to help her was she allowed to be

Only在这里是倒装句.
当句首状语由“only +副词”,“only +介词词组”,“only +状语从句”构成时,句子须倒装.例如：
Only yesterday did he find out that his watch was missing.
Only because there were some cancelled bookings did he get some tickets in the end.
谓语动词放在主语之前的这种语法现象称为倒装.从倒装的形式来看,可分为全部倒装(full inversion)和部分倒装(partial inversion)两种.前者是指整个谓语置于主语之前,而后者仅是指助动词、情态动词或be 动词等功能置于主语这前.
其实这两者没有很大区别,不过如果学会用倒装句,和外国朋友交流起来会更自然的.