Usually. CPAP is the first line treatment of moderate to severe sleep apnea with associated symptoms. However, success rates vary, and a significant number of people need alternative or adjunctive means of treatment.
Very Likely. If that someone has obstructive sleep apnea, and they actually use the CPAP machine correctly, it is very likely to greatly improve or eliminate the apnea. They work very well. Now, some people have difficulty tolerating CPAP and if so, see a dentist who is trained in making an oral appliance to improve your airway.
Gold standard. The CPAP machine is the gold standard for the treatment of sleep apnea, especially moderate to severe. Unfortunately, it is not always tolerated by patients- comfort, claustrophobia, travel problems. Oral mouthpieces offer a nice alternative but discussions with the patient's physician or sleep doctor is imperative.