'has been friends' is correct and usable in written English.
You can use it to refer to a long-standing friendship. For example: "John and Mary have been friends since they were children."
Knight has been friends with Myers for 30 years.
Timilty has been friends with Bill the cat for the past 15 years.
"George is whipping Frank and me," said Rodgers, who has been friends with Hirsch since 1976.
Calcraft said he has been friends with the prime minister, whose electorate neighbours Mackellor, for years.
Graham, dubbed "America's pastor", has been friends with Richard for decades.
Junpei has been friends with Sayoko, and her ex-husband Takatsuki, since his first year in college.
Jurevicius, who grew up in the Cleveland area, has been friends with Zahursky since the two were teenagers.
Ms. Morgan, a singer and songwriter, has been friends and church buddies with Ms. Palin since elementary school.
In the middle of it all was Goldie - born Clifford Joseph Price - who has been friends with TATS Cru since they visited the United Kingdom in 1985.
Howard has been friends with Bryant for four years, their relationship forged when he sought Bryant out for advice on how to get better.
He has been friends with Ferguson even before the two played at Falkirk, when they "were young kids" at Queens Park in the early 1960s.
