Speaking Freely About Motives

How do I know when I have earned the right to speak freely to someone about something that may come across as offensive? 

Not long ago, an acquaintance shared with me that his/her marriage is ending.  (I am now going to dispense with the s/he and his/her stuff....I will use the traditional generic "he," even though this person may actually be a she.  I want to keep the confidence that was placed in me.)

He told me that his spouse had cheated on him in the past, but that he chose not to take advantage of his biblical right to a divorce for various reasons.  I suspect there was some concern about the financial ramifications, as well as the effects on the children, so he stuck it out.   His spouse, who apparently is an unbeliever, had suggested that they divorce on several occasions, but he chose to try to stick it out and make things work.  As he shared his heart with me, I could hear the pain of an unloved spouse in every word he spoke and it made me incredibly sad.

However, the story doesn't end there.  My acquaintance has had a best friend of the opposite sex for quite some time, but recently, the best friend divorced, and now the two of them have developed feelings for one another.  So now my acquaintance is ready to divorce his spouse.

While I totally understand this person's desire to leave an unhappy marriage, and while he has biblical grounds to do so (both the adultery and the unbelieving spouse provisions apply here), I question whether he is doing this for the right reason.  I am very sympathetic to his postion, but the question of how this will affect his children remains, as does the fact that his real reason for wanting a divorce is not because of the adultery or the unbelieving spouse, but because he has found someone else and wishes to pursue a new relationship.  Isn't this adultery on his part, at least in his heart of hearts?

So as I talk with him as he goes through the process of separation and divorce, do I have the right to share my concern about his motives?  And do I share with him that leaping from one relationship directly into another is not a wise thing to do?  Do I share my concerns for his children, and how they may feel about his new relationship if he doesn't allow healing time between his divorce and beginning the new relationship?

I'm praying that God will grant me a great deal of wisdom as I talk with this person, and perhaps that He will move our acquaintanceship to the level of friendship so that I may earn the right to speak freely of my concerns.