"remains friendly" is correct and usable in written English.
You can use it when you want to indicate that someone is continuing to be friendly or acting in a friendly way. For example: Despite the disagreement, our relationship remains friendly.
This in turn allowed the newly fledged Russian government to keep Mir operating, in addition to the Russian space program as a whole, ensuring the Russian government remained (and remains) friendly towards the United States.
He remains friendly with Thomas, whom he hired to coach the Pacers in 2000.
"I can't emphasize this enough," said Kurt Rambis, who worked under Jackson and remains friendly with him.
The game remains friendly, he explained, only as long as everybody abides by the rules of engagement.
That was the reunion with her birth mother, with whom she remains friendly (as she does with her adoptive mother).
Dr. Stern, a lifelong Upper West Sider, said she is divorced but remains friendly with her ex-husband, a professional poker player who settled in Las Vegas.
But the core of the fund remains its Friendly Towns, now in 13 states and Canada.
"He surrounded himself with strong, well-educated people," said Karon Brooks, Mr. Scrushy's former wife, who remains on friendly terms with him.
They are now divorced but Khan, who lives in Fulham and in Oxfordshire with her sons, remains on friendly terms with her ex-husband.
Gilbert, who worked with Agassi from 1994 to 2002 and remains on friendly terms with him, said he also had no idea that Agassi tested positive for methamphetamine in 1997 and was later exonerated by an ATP antidoping tribunal in 1998 after lying about the way the substance entered his system.
But so far, the State Department remains on friendly terms with Lobo.
