PI cutting 2/3 of stipend without notice. Any recommendation?

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Hi guys.

I am asking on behalf of a friend who is in their second year of their PhD in the US. This month, without any notice or warning, their stipend is reduced by two-third (normal stipend is $2300, now is $725, which is way below minimum wage).

This had happened before but never at this extreme rate. In the past, the PI had reduced all PhD students' stipends by $100-200 for a few months due to lack of budgets (sometimes with notice, other times without notice). This month, my friend asked another PhD student in the same lab if their stipend was reduced and it turned out that the other PhD student's stipend was reduced by only $200 while my friend's was cut by almost $1600.

In the beginning of summer, the PI was not pleased with the direction of my friend's research idea and threatened that he would stop supporting their PhD if there is no better research idea or progress. However, in the most recent meeting with the PI (2 weeks ago), both were happy with the new research direction and the PI didn't mention the earlier threat anymore. My friend also just submitted a manuscript for another paper so they were very surprised (and sad, and anxious) about the sudden cut of stipend.

My friend emailed the PI and the associate director but hasn't gotten any reply. Who should my friend contact about their situation (department dean, program director, graduate director)? Is there any legal action they can take? They don't really want to escalate the situation but this is honestly unacceptable. They are also international student so they are not sure if legal action even is an option. If this continues, they plan to just take a few more courses in the Fall and graduate with a non-thesis master and go somewhere else. Is there anyway they can seek assistance from the school if the PI refuse to pay for tuition in the Fall?

Sorry for the long post and I appreciate any suggestion and will relay them to my friend.

Cheers.