The following is a free adaptation of an excerpt from a letter by the Rebbe dated Elul 27, 5715 (September 14, 1954).
… I was distressed to hear of the disharmony in the marriage of your daughter and son-in-law. My feeling is that all that is needed here is a bit of “glue.”
What is glue? Since two hard surfaces do not bond, one introduces a drop or two, or a thin veneer, of a fluid and flexible substance, by which the two hard surfaces are fused together. Employing this analogy, when two people have hardened themselves to the point of inflexibility vis-à-vis each other, what is needed is for a third party to privately speak to each of them individually as to how all their disagreements might be lessened and ultimately resolved through a show of goodwill on both sides. In this way, they can begin moving towards a reconciliation without feeling that they have compromised their dignity and pride.
May the Almighty grant that all the pain and heartache they have experienced in the past should suffice, and that the above approach should yield positive results speedily and without further difficulty; I hope to hear good news from you concerning this matter as well…
 Igrot Kodesh Vol XI pp 417