Well I am not sure if there is much you can do without hurting your fish..... freshwater rays barbs are still reverse serrated just like sw rays, though there may be less serrations in fw rays, it will still cause damage if you attempt to pull it out. As far as the venom in the spine goes.... the only way to help that is very hot water - not something you would put your fish in. Unfortunately I don't think there is much you can do =/
I am not sure what type of ray you have or how big its barb actually gets but you can trim the barb on the ray to avoid something like this again, you can do that with dog toe nail clippers.