When did Sula and Nel's friendship end?

The catalyst for the break between Nel and Sula occurs when Sula goes to bed with Nel's husband, Jude. Jude initially isn't physically attracted to Sula but sees how she could engage a man's mind. He's intrigued by the challenge that Sula seems to represent, which contrasts with Nel's more malleable character. Nel walks in on Jude and Sula having sex and, initially, registers incredulity at what she sees. After they notice that Nel has entered the room, Sula sits up in bed, leaving her nude body exposed. She doesn't look at her shocked friend but instead casually examines her fingernail.
Both Sula and Nel are products of the Bottom, the ironically-named black community in the town's valley. Nel, who is lighter-skinned, grew up with a mother who believed that she could gain favor with white people by distinguishing herself and her daughter as "a better breed" of black folks. Nel witnesses this one day, as a child, when her mother smiles obsequiously at a white man who insults her. Sula, who is darker, is the daughter of Hanna, a beautiful woman known for having many relationships with men. The Peace family, particularly its women, are known for existing on the margins of an already ostracized community.
Despite these contrasts in their backgrounds and their characters, I wouldn't agree with the previous educator's assessment that it's "amazing" that the girls became friends. They grew up learning different ways to cope with oppression. The friendship between the girls is cemented when Sula staves off a pack of boy bullies by slicing off the tip of her finger, warning them that, if she's willing to do such a thing to herself, what might she do to them? Arguably, Sula's decision to sleep with Jude is a similar act. It, too, bears the mark of mutilation and severance. The purpose, in both instances, was to protect Nel, to let her know that Sula would protect her right to exist. It's not until the end of the novel, after Sula has died, that Nel understands her friend and the things she tried to do for her. Sula's transgressive act of "taking" Nel's husband had nothing to do with wanting Jude and everything to do with getting Nel to see which bonds would really help her to flourish.

Sula and Nel's friendship has more than its fair share of ups and downs. But it is amazing that they ever became friends in the first place when considering their radically different upbringings. In any case, as Morrison is anxious to point out, friendship is a complex business, involving good times and bad times: this includes love and hate, loyalty and betrayal. Betrayal is what finally puts an end to Sula and Nel's friendship.
Sula has a different attitude when it comes to forming sexual relationships, one that puts her outside the bounds of so-called respectable society. Nel, on the other hand, subscribes to the social role accorded to her as a young African American woman from a good family. The friends' respective worldviews are diametrically opposite to one another, and it is their eventual collision that leads to the breakdown of their friendship. When Sula returns to her hometown after a lengthy absence, she hooks up with Nel's husband and begins an affair with him. Unsurprisingly, Nel is infuriated by this and severs all connections with her former friend.