This can be a really sticky situation. Handle with care. And love. And respect.
Really. That's my advice.
It's not your responsibility to patch up your SO's relationship with her OSO, or to gird your loins for a big break-up battle. It's your responsibility to meet the situation in a direct, loving, compassionate manner, with care and respect for all parties.
Ultimately, the responsibility for the state of their relationship rests with them, but you can encourage and support open communication and loving behavior.