In law, there is no reason why H&C could not be successfully used to overcome s.117(9)(c)(i), however we do need to think practically as well. Given the legal prohibition, and indeed the moral aversion held by most Canadians, to polygamous relationships, it is unlikely that an officer would grant permanent residence in a situation where there is ongoing polygamy occurring on a factual level. In other words, if the husband is really engaging in polygamy by carrying on with more than one wife at the same time, H&C discretion is probably not going to be exercised absent very extenuating circumstances.
I think you may get more traction, depending on the facts of course, if your case involves a situation where the first marriage still exists on paper only, where those original spouses are not still in relationship with each other, and there are some legitimate reasons why divorce is either not possible or not feasible. You might still have an uphill battle, but it is at least an arguable proposition.
So in sum, the first recommendation to the client should be to dissolve all marriages and either remarry the current partner or establish a common-law relationship with the current partner. If this is for some reason impossible, H&C may be a viable option if the fact situation is sufficiently compelling on a sympathetic level.
For more information on polygamous marriages in immigration law, see: