Can my wife use an ice pack on her stomach during pregnancy?

Yes: There would not be a problem with the fetus.

If needed, sure.: The fetus is more vulnerable to increased than decreased core maternal temperatures. A topical application of ice pack is safe within reason - although I am not sure i see what the indication would be. If it is a muscle sprain, i would prefer oral tylenol (acetaminophen) with an icy-hot patch to the affected area instead.