the answer is easy, DONT DO IT! if they seem like a strong couple leave them alone. however there are exceptions, for instance if you think that he is not right for her or does not treat her good by all means go for her.
skills to approaching a girl
By means of letters and the use of technology now you can easily get someone else to be your boyfriend without personally approaching him.
I would confront the girl or warn your boyfriend
Boyfriend.
Mostly a girl gets a boyfriend at the age 13 or 15
You can understand that by their movements approaching the girl. They will hesitate somehow, will be shy talking to the girl.
Ask the girl.
If a girl keep on talking about her ex-boyfriend to you that means she still loves him!
If he wants to marry another girl he's not your boyfriend.
Then she has to choose you or her boyfriend.
If she has a boyfriend, leave her alone.
That she isn't ready to have a boyfriend yet or she is to busy for a boyfriend.