Thank you so much @nkaravaki for your caring heart towards those whose desire for children has not being fulfilled.
Speaking as one whose dreams have been shattered (and redeemed😊), I commend you for attempting to put yourself in the shoes of one who is struggling/suffering in this manner.
With respect to your question:
I would focus more on listening and being present in their pain. This may involve asking them heartfelt questions geared toward inviting them to talk if they need to, but usually when someone can tell you care they will open up when they need to.
I do believe God responds to “an expectant heart of faith”, but not necessarily in the way we hope. I think we need to be careful that we are not putting ourselves in God’s place when praying for a miracle. As Jesus said in Luke 22:42 “not my will but thine be done”, we should be prepared to submit to His will.
When someone is struggling with unmet dreams, it is a sort of death that must be grieved. Being in the midst of this process is so difficult because one holds in tension a sense of hope as well as a readiness to accept the loss. Most of the time, the person or couple are well aware of their options; the struggle is with grieving the potential loss. This is where being with the person/people and simply loving them is paramount. It seems like you are already doing this.
In reading @1rickolson 's response, I thought of Hannah in the Old Testament who’s sincere desire was to have a child. So sincere was her desire, that she prayed with everything she had and promised God she would submit her son to his service. With the question of desire and whether a desire is God-given or not, I believe that the desire for a spouse and for children are put there by God. However, we live in a broken world and this means that our honest desires may not always be met.
I hear what you are saying:
It is so difficult to watch people we care about suffer through painful times like these especially when they seem to be taking a toll on the marriage. May God grant you wisdom and discernment as you navigate these waters with this couple. May He guide you as to how best to love them through this harrowing time.