A moody friend may be particularly difficult to handle. As much as you may love and value your friend, the mood swing may sometimes become difficult for you to handle. You may feel put off by it, find it irritating, frustrating, and it may make you feel helpless and stuck, not knowing what you can do for the friend so that he/she returns back to normal.
For most of the individuals who are moody, the mood swing may be generated by internal responses to internal thoughts, problems or external triggering factors. It takes their own mental coping, mental processes to come to terms with what is bothering them. Most of the times, not interfering is the best option you have.
Do not try to take the responsibility of the mood swing on yourself. As a friend you may feel obliged to watch out for the friend, take matters in your hand and do your best to restore normalcy in the mental condition of the friend. However, do not try to push them hard to change their mood. Unless they resolve it on their own, your efforts may not really help them, but make them feel more helpless.
The friend going through the mood swing may feel that you are not understanding, not empathizing, being pushy, violating the personal space of the friend. Despite the depth of the friendship, there is need for a buffer space and you must give that to your friend. Leave him/her alone if he/she does not respond to your questions or you get the hint that the friend will prefer to sort it out for him/herself.
At the same time say that you understand the need for their space; and your non-interference does not mean you do not care. Keep yourself open to them whenever they are willing to get back to you.