A friend was just diagnosed with mouth cancer, is that a serious condition?

Yes. Cancer of the oral cavity can take many forms. It may be cancer of the tongue, the jawbone, the gums, the salivary glands, the sinus cavity or the throat. Whether or not it's serious would depend on where it started, what kind of cancer it is, and what stage it's at (how far it has progressed). It also depends on risk factors such as use of alcohol and tobacco, and family history.
Yes. Depending on the location, type of cancer and stage of cancer, it could be very serious. Effective treatments are available for early cancers and your friend should seek care asap. May I add that I would advise against wasting your time on "alternative" treatments.