I am a person who go for facts lor,
so I will give you details of what I have found out .
1) first explore the fact of why the gf likes the boyfriend?
1a) Does the girl love the guy very much and willing to die for
him?
If yes then forget about even telling this type of girl cos it will
not be of any use.
Through the years I have seen this type of gals are willing to
share bf or husband with any girl. What you do will not mean
anything to her.
1b) what type of girl is she ? conservative type, native type,
smart type or the wild type?
1b1) Wild type, no point in telling. Maybe she also want threesome?
hehe just kidding!
1b2) Conservative type, Never have sex before at least she is still
safe from the bf. She still can be save and the only type i feel
that is worth the effort in saving!!!
1b3) Native type. wah this type is the hardest. will believe in bf
no matter wat you say. Got one case even when the gf saw that the
bf is having sex with another gal, keep on saying that they are
just playing around.
1b4) Smart type. just hint can liao, they will know what to
do.
2) Do you think the guy is yao dao and know the art of sweet
talking?
Sadly this is true if the guy is good looking then will attract
more gals everywhere he goes and with his sweet talking skills will
dissolve any attempt from you to break them up.
My suggestion is this
know what type of girl you are dealing with and think carefully if
your advice will have any effect on her. If nothing goes into her
head or love is all that she can think of then do not borther. why
waste time ?
Please read on if you think she can be save from her evil bf or
else do not waste your time.
Ok now that you have decided that she is worth the effort and will
at least willing to act once you have hinted her or say
anything.
Then this the part where I will teach the art of hinting :
1) Girls are very sensitive people. hehe! Oi relax lar, this is not
some XXX story. They are prone to hearsay. just spread among her
friends that you saw her bf with girl and holding and kissing the
news will spread.
2) If can try to take photo of 2nd gf with the bf. The next time
when he goes batam. You invite her and her friends out for ktv or
wat. The idea is to slip the photo into her bag without her
knowing.
3) you know her email rite so just use an unknown email address and
sent the photo to her email address.
4) Use a new number to sms her the full details of her bf
misdeed.
5) This one is the killer blow: Paste the A1 size photo of the bf
and the 2nd gf have sex or kissing on her front door. Then siam
asap. This method sounds like a loanshark rite? hehe But the effect
is the greatest cos her mama and papa will for sure kick the bf ass
out for you.
Final Thought 
What I have say here is to be use with caution.
You got to think carefully first.
I can offer only ideas thats all.
The rest is up to you to decide what to do.