Let them know that regardless of what time of day or night they need a shoulder to cry on, you are available.
Let them know you will listen silently if that is what they want. Sometimes there are no words and all of them become difficult to hear.
If you cannot offer that, then possibly helping them find a group they can join for parents in their position would be the best option.
To this day I have yet to hear one person tell me I can scream, throw a fit or just cry and they will be there to hear it.
Truly it is the one thing I wish I had access to just once, especially from someone who truly cares for me.
Instead, people keep their silence for fear of hurting me while others speak things I honestly wish they hadn't. I try to appreciate them for trying, but some people just don't stop to think about the words they speak. My all time favorite (sarcasm) is, "You can have another baby someday!"
Many words are time dependent too. One day a parent may be able to hear them, but another day they may strike their core.
Loyal support is the only safe option I can honestly think of. I hope this helps.