In my opinion a friend should be like the old adage which says 'A friend in need is a friend indeed!'
Thus a friend who is always there for you in your happy moments and sad moments.
He/she is there to share the happy moments and there to give his/her shoulder to mourn at the time of crisis.
I personally feel a friend should be trustworthy with whom we can share our secrets, he/she should be sincere towards the friendship, he/she should be honest, he/she should be understanding and most important is that he/she must be caring!
One would be really lucky if he/she does have a friend with all the above qualities!