The phrase "the boyfriend of" is correct and can be used in written English.
You could use it to refer to someone's romantic partner in any context, such as: "She was talking to the boyfriend of one of her friends."
Patterson is the boyfriend of the child's mother, they said.
Satko Mujagic had been the boyfriend of Karabasic's niece.
The boyfriend of Peter's mother is jailed for life.
The boyfriend of the woman on the bench arrived, carrying iced coffees and breakfast sandwiches.
The boyfriend of a mysteriously unmarried aunt had disappeared for ever in 1965.
This is Elliott, and he is the boyfriend of Hannah Reid, London Grammar's singer.
Hugh, by extension, was granted a visa as the boyfriend of a writer.
The boyfriend of Hemmingson's cousin had called Kargbo a nigger in front of Destanee.
And the boyfriend of eight years was gone and, with him, the Fifth Avenue party pad.
The boyfriend, of course, had moved into the apartment the moment I left.
This house reportedly belongs to the boyfriend of his mother, who is a schoolteacher.
