Why is Helena Charles not the moral compass of the play?

The short answer to this question is that Helena has an affair with her best friend's husband. Unfortunately, this is another manifestation of the air of cynicism that pervades the play. It's therefore somewhat unfair to single out Helena as lacking moral authority, since it's doubtful that anyone in Look Back in Anger could fulfill that role.
Osborne shows women to be manipulated by Jimmy. In spite of his abusiveness, Alison stays with him. The underlying theme may be that people have needs that are actually detrimental to their interests. This would include Jimmy himself, because he seems self-destructive, preferring to maintain a dysfunctional dynamic in his home life. His obnoxiousness, one would think, is such that he not only makes those who are with him miserable but does the same to himself. Helena at least makes an effort to help Alison, and the fact that she also becomes mesmerized by the supposed allure of Jimmy is probably meant to show the irrationality at the root of much human behavior. It is also not difficult to see, by our standards today and perhaps those of Osborne's own time, a male chauvinist agenda underlying the action of the play. In any event, we can at best evaluate Look Back in Anger as an at least partly honest description of an all too real dynamic that exists in many marriages and extra-marital situations.