Mad Dad,
Just in the interest of considering all options, here is a link to the Tri Sigma website where it describes some alternate membership options. Scroll down to the bottom. There are some options for inactive status where no participation or dues are required.
Link:
https://www.trisigma.org/commitment-...ge-and-beyond/
Otherwise, if she can't find a website link as described previously, perhaps your daughter can phone the headquarters directly to request the termination.
Best wishes to you both.