Whilst it's difficult to do, the answer is however very easy.
From the No. 1's perspective, if she has been approached by her crew, then that has to be regarded as an official approach and it is her responsibility and duty to report the situation to her boss (probably the head of cabin services). The head of cabin services then has the responsibility to report it the matter to the Dir of Operations. You can't expect the No.1 to have to go direct to the Ops Director.
Likewise, any F/O finding him/herself in the same situation has the same responsibility. Again, not easy. If an F/O turns up for duty and it is obvious that his colleague is not in a fit state to fly, he should not go to the airplane with him. If his colleague insists then he has to make it official there and then and should refuse to fly. No-one is going to sack you for taking the right, responsible course of action.
Attitudes to drinking in the workplace have changed significantly over the years, and whilst it may have been viewed as a 'bit of a lark' that most of us have been guilty of at some time or other in the past, it is now socially unacceptable to fly, drive or operate any kind of machinery whilst under the influence that could render injury to the operator and others under his care.
Likewise, most managements today also recognise that those who do it probably need help and should not be ostracised.
No one wants to 'shop' a colleague, but if it helps to muster the courage to do what you have to do, remember that you are probably doing the person a favour. He or she might not see it that way at the time, but when 'cured' will probably thank you. They are a danger to themselves and maybe cannot help themselves.
To ignore the problem is to condone it and that implicates you.
I'm just a bit curious how the 'observers' know this is happening up to 2 - 3 hours prior to report? Shouldn't they be in their pits?
On a lighter note, I still smile at the alleged court case of a B727 crew (quite a few years ago now) that got reported by a member of the bar staff from the establisment they were drinking in the night before. When questioned, the waitress said, 'I knew he was drunk, because when he tried to stand he fell over'. The accused lawyer supposedly said that '8 pitchers of beer doesn't affect my client as it may other people, because as an alcoholic he is used to consuming larger quantities of alcohol than most and can handle it!'. Supposedly true. That was the typical kind of attitude many years ago, not true today.
Joking aside, for the benefit of any non-aviation professionals reading these pages, I honestly believe that such instances today are miniscual and nobody should read this thread and believe that it is a widespread problem. These problems exist in all walks of life and to a far greater degree in any other walk of life. Take comfort from the fact that the crew of an airplane is not one person, but many and the chances of them allowing one person to act in such a way are very remote. That is why the other crew must never turn a blind eye to such things.
I hope that helps your friend.