This is a sad situation, for sure. Is the news of her illness common knowledge and with her consent?
If so, expressing your concern for both your boss and his wife with an offer of help is a lovely way to show support.
This can be done in person through the husband or via card to the wife. A simple card letting her know she is in your thoughts is always comforting. Because of her condition, one does not know if flowers would be helpful or if she is allergic, etc., so I would forego the flowers and send a card or personal note. Depending on how familiar you are with either the husband and/or the wife, you may want to write a note every so often to remind them you are supporting them in this difficult time and as once again if you can help.
The bottom line is that anything you do that shows your care and concern will help them.