You can't file for a change due to "decreased income"; it is only for a "life changing event".  See the top of page 2 of Form SSA-44:

https://www.ssa.gov/forms/ssa-44-ext.pdf

But in a couple of years, they will look at your client's lower income and decrease the premium based on that year's AGI.   If your client's income goes back up, so will the Medicare premium the next year.