How can exes fix things?

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So my sister and my friend broke up few months back. It makes me sad because I'm in the middle of it all, and right now they're both just so damn stubborn and hate each other. They broke up as a lot of things (outside the relationship) was getting in the way, causing problems for both, just life in general and they slowly drifted apart. But they were always perfect together, nothing bad actually happened between them, together over a year and each others firsts, love of their lifes etc.

Anyway it's been about 3 months, and they were arguing a lot but now my friend completely ignores her now for like the past 3 weeks. She acts happy about it but the problem is, she's now just going from guy to guy, I know she's trying to find that connection again. I read her messages with her new boyfriend and she's sending him loads of kisses saying she loves him he's EVERYTHING to her (some guy she's known about 2 weeks). She said the same stuff to few other guys in the past 3 months she tried getting with. While my friend, the guy, he acts happy online but he's actually really depressed. So how can I be there for my sister? I don't want her going from guy to guy to try fill a 'void', I've tried talking to her about fixing things and just a month ago she said she might do, but now she's gone back to saying she don't like him anymore, yet if that was true, she wouldn't be acting so eager and desperate with other guys.