User blog comment:B214/Chapter 383 Surfing Lucy thoughts and analysis/@comment-3424593-20140509123200/@comment-7418318-20140511225656

Well... she mostly appeared to bandage the deep wound in Lucy's left arm from Gemini wielding Caleum as a sword against her. The change of clothes was just an added benefit.

At least in the manga that wound certainly looked deep enough to bleed Lucy to death if kept untreated.