It is definitely something to be concerted about for Hardy, heart arrhythmia can be a very serious thing. I suffered (and still do to an extent) a very bad arrhythmia, I would consistently pass out from it and it would cause me major distress, it can also cause cardiac arrest. I had to go through multiple ablations and medication to finally tone it down to where it is now.
Point being, if the doctors feel it can be a serious thing then it really is not safe for him to fight. With that said, if there are no symptoms and if he's not having any real problems, I don't see why he couldn't be allowed to fight up until those symptoms (if they ever do) start showing.
Hopefully they let him fight if that is the case.