Without the proper OK from the US Government, a trip to Cuba could be illegal and might compormise your wife's status.

That aside, many US citizens do go to Cuba on holiday and as long as they do not talk about it to US customs on their return, have no problems.

You will need to make sure your passports are not stamped, and beaware that US credit cards and traveler checks (issued from a US bank) are not valid in Cuba.