you can but try though!
Epsom salts will help with the retained fluids but will not cure the cause.
From what I understand about dropsy, the condition itself is not infectious but the causes can be , if that makes sense.
i.e Dropsy can't be caught.. but a bacterial infection that caused dropsy can be. So it comes back to cause.
I would isolate the sick fish, treat with Epsom salts and a systematic antibiotic like furna2. (weigh up the medicine cost vs the cost of guppy and likelihood of success which could be quite low) You may be too late to save this one but keep a close eye on others.