Epsom salt do seem to help thoughSalt in the water isn't going to make or break to soak.
Soak her in warm water and gentle massage that area.

@Lilith_Luna she needs a soaking, then try to remove all the stuck poop you can. Is there any way that you can dig that out? Sounds gross, but a while back someone used a popsicle stick to gently work dried up poop out of a vent. You would work slowly, remove what you can and re-soak the hen. Let her rest in between if she is stressed.
If she cannot poop and is plugged up, she won't make it.