how do I get my friends to understand I want to help their relationship not ruin it?
I also love the boy that the girl is with so yeah
There's nothing for them to understand, it's their relationship. You need to stay out of it, especially if you have feelings as well.
Watch How I Met Your Mother. It should help.
I've been in your situation. the best thing to do is to stay out of it. trust me. you don't want to get involved.