What happens to a section 8 voucher if it was granted using two incomes now one person decides to move out?

My grandmother lives in a two bedroom section 8 apartment with her brother. They are both seniors and it was my grandmother who applied and got the voucher. She listed all household income which included her brother's retirement check. The rent they pay is based on both their incomes. After a year and a half, her brother decides that he doesn't like having a room mate and plans on movie out of town to live near one of his daughters. Now my grandma faces the household income being cut in half overnight. She just signed her new lease 3 months ago. It's a 2 bedroom and obviously now the rent is twice what she could afford....not to mention all her other bills have just basically doubled. What will happen to her? I know she won't lose her voucher because she's the head of household but will she have to stay in that apartment paying her current rent amount? Will section 8 pick up the difference for the remainder of the lease then make her move to a 1 bedroom? Will she have to move into a 1 bedroom immediately? to complicate matters a little more, it's technically a condo so she deals directly with the owner, so it's not as easy as just moving into a one bedroom in the same building. I know I can just call the housing authority and ask, but he hasn't moved out yet and I don't want to start anything that may make things worse.