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Hi Michelle. I'm very sorry to hear about this situation. Were these two in a very warm area for a longer period of time without water? Had they been completely normal earlier in the day as far as eating / drinking / behaving? Any chance of any chemicals or things being burned on a stove near them? It is very good that he surviving one is eating and drinking on her own. For the most part, the best that can be done for her at home at this time is to have her in a quiet area where she can be non stressed and let her body recoup from this. As far as hydration at this time, if she's drinking on her own, I would just have plain tap water available. Will she want a new partner? That is something that more than likely she'd benefit from as far as having the social interaction helps to stimulate them in a positive manner.
Sun conures are very social creatures. The addition of a possible new mate though shouldn't be rushed. She's lost her current mate and it is expected for her to be grieving for that one. She needs some time to go through this process. To rush and get a new one immediately would expose her to this new individual at a time when more than likely she's not going to be as receptive. Here is a link to a short write up about getting a new mate that goes into this with more depth.
I would hope that at this time it would be ok to be back in the cage. As long as there are no exposure to fumes / chemicals, it is in a controlled environment with good food and water access - then it would seem ok. One of the things that can look similar to this is if a non-stick pan is burned on the stove as it can release a very toxic chemical for birds that can lead to acute passing.
The best thing that can be done is for the both of you to interact with her as much as possible to try and give her the social stimulation she needs. If she gets a chance to interact with you both, then it will hopefully keep her well stimulated.