Be honest-no matter what.
Share your feelings-generally speaking we aren't mind readers.
Be considerate of other people's mistakes and quick to forgive them for unintentional harm.
Be aware of your own mistakes and be quick to apologize for them.
Love freely with one another.
Let each person find their way. In your situation there is THEIR relationship, your relationship with HER, your relationship with HIM and the relationship the THREE of you share. YOU can not control the growth or demise of THEIR relationship. Likewise neither of them can control the growth or demise of your relationship with the other, but all three of you MUST work together to SUPPORT each others relationships and all three of you MUST work together to support and grow within the relationship of the three of you.
"Love As Thou Wilt"