It is the responsibility of the gambler to keep safe guard his system and also not to click on any malicious links. Gambling sites are only responsible for any glitches that occur in the gambling site, which did not happen in this case.
I think a problem like that is a bit strange indeed, it's true what you say, we also won't know if the user might lose the bet, so this doesn't prove anything to say that Stake has a problem because it's all the user's responsibility, it should be if it's an email, why not? verify it on this forum to find out more about whether an email is safe from phishing or not, sometimes laziness can also harm ourselves, I think this will be a valuable lesson for the OP to be more careful in the future.