You can find the 17-page opinion here. Among other things the court found that the limit on third party collection of ballots would suppress voter turnout of Native Americans.
This is one of the Marc Elias lawsuits (press release here). I don’t know enough about Montana state courts to know if this ruling is likely to survive appeal. But to the extent it is based upon interpreting the state constitution, this is not a case likely to end up before U.S. Supreme Court review.
While I believe Elias is bringing a lot of important litigation, in this piece I am skeptical of some of the lawsuits raising claims like the one in the Montana case—except to the extent, as perhaps here, of showing racially discriminatory impact.