Although we worked really hard to disallow “cross-over” voting in primary run-offs, legally, your niece has the right to vote in either party’s run-off primary - if she did not vote in the primary election. I think this was a better solution than requiring voters to register for one party or the other like in some states.
I don’t fault AL for what idiots do thinking that spite is ever good. I told her meanness will make one ugly. Look at Hillary and Pelosi. I may be off the Christmas card list.