The two main dangers of drinking alcohol (assuming you're talking about small amounts of social drinking and not severe intoxication) are the sedative effects and the effects on the liver.
1) Many medications can cause sedation including, among others, pain medications, anti-anxiety meds, anti-nausea meds, anti-histamines, some psychiatric meds and sleeping pills of course. The danger is that the sedative effects of the alcohol will be added to those of the other drugs so you'd sleep much too long or even, in extreme cases, stop breathing altogether. So while not specifically dealing with the chemotherapy, this can be a problem with other medications that the patient may be taking.
2) The liver is necessary to clean the body of many medications, including chemotherapy. Alcohol can interfere with the liver's ability to do its job and the patient can suffer. For this reason, when in doubt, don't drink alcohol.
3) After considering #s 1 and 2, it is important to realize that alcohol is an important aspect of both relaxation and socializing in many people's lives. So ask the oncologist whether in the patient's situation it is safe to have the occasional alcoholic beverage.