This can be a frustrating situation but avoiding feelings of jealousy and anger is a priority. You do not want to come barging in and screaming at the boy and girl.
My answer to the related question (provided in the Related Questions Link) describes what you can do instead.
Hold her hand or put your arm around her waste :)
you say, " so hows it goin' between you and your girlfriend?" .
Dan wrapped his arm around his girlfriend's torso.
Maybe he likes you! If you really like him you gotta act confident! Show him that you are way better then his girlfriend! It probably means that he likes you. Except him breaking up with his girlfriend soon!
He will put his arm around your shoulder and get real close.
-If you two are walking together put your arm around her waist
Hold her hand or put your arm around her or just walk with her normally and talk.
Yes. Especially if the boy feels uncomfortable doing it first. My girlfriend put her arm over me first.
Relax it means he likes you, you can even put your arm around him.
Just gently put your arm around her and just slowly pull her close to your chest
he may like you he may like you
From personal experience, I like to put my arm in front of my girlfriend's arm when we hold hands, but that is probably because I am taller than she is. You should try different ways of doing it yourself and see what is the best for you two.