It is neither appropriate nor justified, but it seems as if the situation was desperate enough for all parties involved for it to end the eventual dismissal of the mistress followed by reconciliation. I was a bit shocked when I learned the end, myself, because he had been cruel enough to Alison for her not to grant him any privileges, and his treatment to her was abismal. Since this is an account of Osborne's fictionalized tribulations, I imagine he really had intended to make this a form of catharsis rather than a mere story that teaches a lesson.
The ending is by no means satisfactory. It's very much perceivable that the final reconciliation is a forced one; not out of the real emotions, nor out of the changed attitudes of the character involved.
Helena, though a considerable match with Jimmy's teperament, was actually a misfit.... As for Alison, I think, after losing her child she also was in need of a substantial support which Jimmy was, regardless his inability to compromise with Alison (broadly, with the middle class virtues).
That the final unification is not a spontaneous outcome of their relationship, is likely to appear so perhaps to everybody, this sort of helpless acceptance (of a large impersonal force; in this play the economical social condition of post-War British life and other aspects wherein the source of the very crises are alaid) is the very motif of post-modern literature and both, Jimmy and Alison, are seen accepting their destiny with a forced instinct... and this is why the character of Jimmy tends to become a "grotesque character".
The ending does not do justice to the play. Helena's decision to leave Jimmy is as unacceptable as her earlier decision to stay back with Jimmy after Alison's departure. Perhaps Jimmy accepts Alison because she is now shattered, and she goes down on her knees before her rebellious husband. Is she now expected to show the 'enthusiasm' that Jimmy had always demanded? The ending is just conventional and populist.
I agree with Post #2 that the ending was more cathartic than anything else. I think that that ending could even be more appropriate because it leads to shock for a more dramatic ending such as what would be expected in an ending that his happy, or ends with success.
The British novelist Fay Weldon offers this observation about happy endings:
"The writers, I do believe, who get the best and most lasting response from readers are the writers who offer a happy ending through moral development. By a happy ending, I do not mean mere fortunate events--a marriage or a last-minute rescue from death--but some kind of spiritual reassessment or moral reconciliation, even with the self, even at death."