There are things best never said to a widow…
Don't let this put you off talking to a widow – hearing the wrong thing is better than hearing nothing at all. Tell them you care, that you are there for them (and make sure that you are - see What can I do to help?), and listen to them. Listening is often the most important part.
Worried you'll say the wrong thing? Read this awesome piece called How not to say the wrong thing.