my cardio says you have to be careful with natural remedies as they don't have to list all the ingredients of which some ingredients can, bring on an episode.
i only happened to ask as i'm sick of meds side effects (mostly the vivid dreams)
if you find a good one let me know.
As far as I know there are no proven natural remedies for SVT. Has your friend been to see a cardiologist? There are some safe medications that are sometimes prescribed to help with symptoms.