According to her best friend, Tameka Foster was the last to know that her marriage to Usher was over.

In a recent interview, Nyeisha DeWitt revealed that Tameka only learned Usher had filed for divorce after she read about it online.
 
“She didn't know that he was going to file,” she told Essence.com. “Of course, she knew that there were challenges in their relationship and had been some discord since March and was aware there were some things that needed to be worked through. But did she know that divorce was imminent? No. It was a total surprise.”
 
Nyeisha, Tameka’s friend of more than 20 years, also disputed rumors that the couple has been separated for months. As proof, she cites the fact that she attended a birthday party Usher threw for his estranged wife back in January.
 
“When I read that people were reporting they'd been separated for a year I'm thinking, ‘That's insane,’” she added. “That would mean he left her when she was two months pregnant with Naviyd, and Usher Raymond, V, was only five months old.”
 
But, regardless of the way she found about the divorce, Tameka still loves Usher and she’s having a tough time with the fact that he wants to leave.
 
“You're married until the divorce is official. Just because she's going through this process doesn't mean that the loyalty and love she has for him is gone,” Nyeisha explained. “It would be dishonest to say that she doesn't still love him and isn't hurting.”
 
And, before she wrapped up her statements on the matter, Nyeisha had one final comment to those that had been hating on Tameka.
 
“These bloggers talk about her as if she has personally harmed them…and to discredit her as a mom or wife is insane,” she said of her friend. “My hope is that people will not rejoice in anyone's failed marriage and allow Usher and Tameka to go through their process privately.”

-- Sonya Eskridge

Here’s more:
Usher files for divorce, Tameka gets lawyer
Usher and Tameka splitting up?
Usher inspires kids to BE ICONIC
Tameka Foster coherent after surgical scare