we have to get married sometimes

Well I think whenever you are with both of them you should not talk about marriage and talk about something else because the one who got divorced might be affected because of what he or she's going through. You could talk about marriage to the one married. About their plans after the wedding, their honeymoon plans, about kids or anything because as of now he or she is still in cloud nine and might have a lot of stories about what they did, what they'll do, the gifts they've received, etc. But when you're with the one who just filed divorce, it would be better to let him or her talk about his or her feeling, lend a helping hand, just listen. I think he or she will need someone who'll listen so he or she could loosen a bit. After sometime when he or she had moved on you can talk to him or her about anything under the sun, even about being married again.