As I posted in another comment, the problem you describe exists for this person regardless of whether it involves an ad or not. The entire UI for this app is a serious problem with a person of these limitations.
I fail to see how going to an ad page is a worse offense than going to Drafts as opposed to Inbox. Either way the person in question has to correct the mistake. Expensive in what way? Money, time, embarrassment? I don't see the difference.
The ad is between Compose and Inbox, so what? Are you suggesting that Yahoo somehow benefits from an accidental click on this ad? What's the purpose of them putting it there? Could it be possible they have it there because they consider that you are more likely to see it there and not that you'll accidentally click on it?
And again, what's the optimum distance that's acceptable? Fifty pixels? One hundred? I would have to assume that if this is the problem then it is a potential problem for nearly every website and application ever made. The reason this bothers me is because people are saying the burden is on Yahoo to fix this when the problem does not necessarily lay with them, but other factors are in play they have no control over. Some people are even suggesting nefarious reasons for this ad placement to "trick" people into clicking on it. For what benefit?
I too sometimes click on something other than what I intended. Most of the time I don't blame the app, I blame myself. There are times that people will employ tricks to get you to do what they want, that's when you complain; this is not a good example of this.