The sources found don't give a detailed account of the ending, so this section sticks to what is documented. The story is set on a deadline: Mary has three days before her comeback show, and Sam has to finish the dress in that time. As they work, the past between them comes out, including their romance, their creative partnership and whatever caused them to split.
According to IndieWire, the emotional resolution is not a clean reconciliation. Sam's anger turns into awe, and by the end her love and hate for Mary are inseparable. Seen after the film, the dress-making is the relationship itself: one more round of the same exchange that made Mary famous and left Sam hurt. Beyond that, the exact final events weren't confirmed in the sources.