Federline spent an hour and a half with Spears at the substance abuse rehabilitation center, said People.
A source that is reportedly familiar with Spears's situation said that Federline has been involved in her stay there.
"He cares a lot about her," the unnamed source said.
Spears's father, Jamie, told FOX News on Thursday that his daughter is "a sick little girl." Adding that, "We're just trying to take care of her."
He said that Britney's problems are "not about what other people think" and that he and his wife are "concerned about our daughter."
Spears's manager, Larry Rudolph, dispelled rumors that the pop star was on suicide watch during an interview with Extra Friday.
"The story is absolutely untrue. She is in rehab, and it is very disappointing that various media outlets are running these false stories about her, as Britney tries to get better," Rudolph said.