I'm sorry to hear of this.
You need to check with the doctor whether supplementary oxygen would allow her to fly. You may need to find an aviation qualified doctor in Alicante.
Then you need to find an airline that will either:
- Allow her to use her portable oxygen aboard
- Set up an oxygen dispenser at her seat. There is only one carrier in the USA that offers this service and is the only one my uncle can fly on. It takes coordination with the airline's medical department some days in advance.
You may want to check with the medical department of the airline that flew your aunt to Alicante.
Hope this helps.