Alas, this is a familiar sight - and not the same as the more common problem of brown edges. It could be a fungal infection, often related to root-rot. Or it has to be if you are able to rule out all environmental stressors (incl a plastic pot). Stressful situations (temperature drops, erratic watering, dry, central-heating air, less light in winter, but also absence of its favorite carer!) weaken the plant’s resistance. The spores of moulds come in through open windows or on your clothes etc. They also hibernate during the winter, and may have been picked up by the plant itself during its stay outdoors in summer; now spring is (sometimes!) in the air, in your neck of the woods, these moulds are reactivated. If you find out what else it could be let me know!
• These seem to be the options: phytophthora, sunblotch, anthracnose, verticillium wilt, stem-end rot, sooty blotch, asphyxiation, albinism, sectoral chimera and ring neck.

In cases of fungal or bacterial infections, the plant needs to reestablish its own healthy boundaries (against intruding pathogens) and learn to trust its own vigour. After removing affected leaves, I am treating my avocado with Bach Flowers: think of Rescue Remedy but possibly add Centaury (some prefer Agrimony), Chestnutbud, Crabapple, Walnut for fungus and bacteria.

To complicate matters fungus thrives on humidity and higher temperatures! So in that case, the solution is not as easy as recreating the avocado’s natural habitat! In any case, ventilate really well (moisutre must be able to evaporate rapidly).

Otherwise, if it is merely general wilting, due to a combination of dry air and insufficient thorough watering alternated by drying-out plus the shock of the cold. Too frequent watering could lead to root rot (also caused by a fungus). The Bach rememdy in this case is: (Rescue Remedy +) Olive, Gentian, Gorse, Mustard + Sweet Chestnut or Wild Rose, for general wilting (1dr each to 30 ml of water, sprayed and dripped on soil with loving care 2x a week for a couple of weeks.). More on such treatment here.

In either case, after checking the condition of the roots (as unintrusively as possible, preferably in spring) I would go easy on the fertiliser while the plant is recovering (won’t be able to metabolise the feed and it could burden it all the more). Soak 2x weekly then let dry out (best in clay pot). Humidify.

Another thing to consider, seeing as the two of you are so close, is whether your plant is picking up on your stress (ill health - eliminative processes; muscular lesions) - or problems assimilating; emotional tensions; mourning; or new electro-magnetic stress in the house?

Avocado wants to open the 3rd chakra (solar plexus). It does well around sunny dispositions and less well in overly rational places (not a good library or stuffy study room plant…).