Doctor my friend has got cavities. ..the dentist told that the decay is too deep nd she needs to have a rootcanal...will a rootcanal be painful? .

No. It is painful when you need a root canal. It it is painful if you let the infection go out of hand so the dentist can not get you numb properly. A root canal is not painful at all. The area may feel soar or tender after the treatment. .
Root canal. Tell your friend to have the root canal done now while the tooth is not painful. The root canal can be done easily with little discomfort. If your friend waits until the decay progresses and the area is swollen and there is an acute infection, then the infection will cause pain. The root canal is not painful, but an infection can be.
Not at all. Modern dentistry is painless. After the anesthetic is administered she won't feel anything.
Very comfortable. With modern techniques and great anesthetics, most patients report that they are very comfortable during the root canal procedure.