If we assume your wife's portfolio is solely that TFSA, it would probably be a good idea to diversify outside of solely the Canadian market. However, there is an element here of her being retired, and likely spending most of her money in CAD, to have a more Canadian bias due to the need of Canadian currency.
So, this is actually where I think CDR's could come in handy. You can get exposure to the US stocks in Canadian dollars, getting dividends in Canadian dollars. The only added fee here is the 0.6%~ annual hedging fee from the CDRs. However, if your wife is retired and doesn't really want to deal with currency swings, hedging isn't all that bad of an idea anyways.