If Ceri is managing her own wardrobe
1. Do not say "FOLLOW ME" if she is not wearing her black/white Vidilia dress outfit.
2. Do not tell her to remember any outfits (to avoid unable to find outfit, Skyrim leaves her naked).
If you already did (1) when she's wearing something else.. the next time she changes to the black/white dress, dismiss and recruit her to "set" that outfit as the default one. Or you can "set kcfDebugMode to 1" and set her outfit as the black/white one, then dismiss/recruit her.
Explanation - The Vidilia black/white dress is the only "real" (but invisible) outfit in her inventory. The rest are dynamic. They don't exist if she's not wearing them.
If you are choosing what she wears
a. Tell Ceri "I will give you nice things to wear"
b. Give her armour like a normal follower and equip her as you see fit.
c. When satisfied with the outfit, DISMISS CERI - "We must part ways now"
d. Recruit Ceri Immediately to tell Skyrim to remember this outfit. - "Follow me I need your help"
e - Do NOT tell her to remember any outfits! This will cause armour nuking if you are managing her wardrobe.
f. After every "upgrade" of armour items, repeat step C and D (helmet, chest, boots, gloves. Can ignore jewelry)
Follower overhaul mods can't fix this bug as explained in my "wall of text" post. Even AFT can't, at least as of v1.5x. If AFT v1.6x has active wardrobe management functions (ie scripted and running all the time), then it could probably fix this naked bug. You must then tell Ceri you will choose her outfits and configure AFT to handle it.