It's one of those ones where you really have to think about it. The frog tells her that whatever she wishes for, her husband gets that amount times by 10. But there's nothing that specifically says which direction that times by 10 works in. The assumption is it must always mean an increase in amount, but that's not actually true, it just means that he gets 10 times whatever she has wished for.

For example, if she wished to have the smallest gold bar possible, then her husband would get a gold bar 10 times smaller. Which means he would have a gold bar a tenth of the size of the one she has. So, even though it is times by 10, the result is less rather than more.

She wished for a mild heart attack, and due to the way the English language works, 10 times that amount means the reward for her husband, is to have a heart attack a tenth of that amount. So, just as if she had asked for the smallest gold bar, which would mean her husband would get an even smaller one, she asked for a mild heart attack, which means her husband would get an even milder one.

Hope that helps! And yes, the Sherlock Holmes one is amazing lol!