Can diphenhydramine and dextromethorphan taken together?

My husband has the worst cough from his cold. Its only bad when hes laying down trying to sleep. The robitussin i gave him didnt work but another one called tussin CF worked. He naturally has problems sleeping so he usually takes 1 tablet of diphenhydramine sleep aid every couple of nights. He didnt sleep at all last night from coughing and has been exhausted all day. Is it okay to give him 50mg of the sleep aid? Its only one tablet.