What to say to friend going through a breakup?

I think a generic variation of 'it sucks' and asking them how do they feel would be enough. 'I don't know' too.. Remember that you don't have to be necessarily the friend who says the right thing, you can be the friend who distracts them, brings them cake and dvds, etc., or the friend who listens, and pats them on the back while they cry.
It's enough to be around.

Be there for that friend. Be a listening ear, a distraction. You can get that friend out of his/her house at times to go do something together that could be fun. It's okay to let that person know that you're there and you can offer to listen, but don't keep reminding that person about it. Avoid to bring it up time after time. If that person brings it up himself/herself then let him/her talk. If that person cries, you can offer your shoulder. If that person tends to wallow in the sadness too much, you should try to distract that person. Just being there would already mean a lot to that person.

well if your friends with both of them tell them each individually that ti doesn't matter, there are other people out there that can make you happy

It's a situation, that depends on the person, and how the break up went, you can really ask them what they want to talk about or do, they will be upfront and Just be there for them if they want to let some emotion out, don't hesitate to comfort them.

Just keep being their friend and be there for them. If they want you to say something to them you'll be able to tell, and you'll probably know the right thing to say