、。
She's outgrown her favorite dress, so it needs to be altered.
Literal
She [topic-は] grew-up [and-て] until-now was-wearing favorite dress [subject-が] can-no-longer-wear [because-ので], it [object-を] must-alter.
A complex sentence packing several grammar points. 今まで着ていたお気に入りのドレス is a long relative clause: 'the favorite dress she had been wearing until now.' 着れなくなった means 'became unable to wear' — the potential 着れる + なくなる (ceased to be able). 仕立て直す means 'to alter/remake' — 仕立てる (to tailor) + 直す (to redo). ~てやらなければならない adds obligation: 'must do it for her.'