Why did page take in Helen ??

I think there were several reasons "he" was interested in the girl. 1) Hubert would have likely been unaware of what a brat Helen had been -- to Albert or anyone else for that matter. 2) Albert approached Hubert when he with still grieving the loss of his wife -- and possibly not yet ready to move on. Since the baby has been born in that final scene it is 6 months to a year maybe since Hubert lost Cathleen. 3) Albert was inviting Hubert into his dream of the future (a shop, etc.) as opposed to building a future together. (Note: This was a rather graceless approach by Albert--a habit he showed throughout the film, seeming to lack or have very rusty interpersonal skills.) 4) As far a Helen is concerned, perhaps she was more Hubert's "type" than Albert was. 5) I imagine Hubert was at a point in his life where he was ready to have a family. This was about as ideal an opportunity as he was likely to find for getting one. 6) Though it's unclear whether Hubert knew of Albert's interests in Helen (I think not), I do believe he had a great affection and sympathy for Albert -- a kindred spirit. Learning that the child had been named for his late friend may have touched Hubert greatly.

That's all I can think of -- but it may be enough to answer your question.