Regarding matters of the heart, there is no rights or
wrongs.
Since you said that they are young people, I assume they are
secondary school teenagers. At such a age, they probably are not
mature enough to see the implications of being so close to their
boyfriend's friend/friend's girlfriend.
Some may argue that there's nothing wrong in being friends, but
there's always a line to draw between just friends and something
more.
There is no right or wrong. To summarise what I've just said, it's
simply bullshit 
