How can I help a religiously committed friend who suffers from nervousness and quick temper, and who admits it but doesn't know how to get rid of it?

The cure for anger is to avoid its causes and remember its consequences, as the Prophet, peace and blessings be upon him, said: "Do not get angry." Anger dissipates by changing one's state, such as for a standing person to sit down, seeking refuge in Allah from Satan, performing ablution, and remembering the virtue of suppressing rage.