Im just really sorry youve had to go through this.
What do you say to someone who lost their daughter. I love you and am here for you Acknowledge when you dont know what to say. You may even fear saying the wrong thing. Attend the funeral if at all possible.
Daughters are your forever little princesses and apples of your eyes. The person who lost the child may be going through a wide variety of emotions. You would give them the world and give up your life for them.
It was so powerful just being witnessed. For example you might be afraid to upset the parents by bringing up their daughters. This is a good sentiment to express when a friends loved one dies from any cause including suicide.
If you go to temple or church with the person every week and youve had deep and meaningful religious conversations with the person for several years and the person would consider you a soul-mate feel free to say something religious like If that child was only going to be here for a short time maybe God wanted it to be with you If ANY of those things are not true DONT SAY. If your friend or family member recently lost a child. If the person wants to talk listen.
Simply offer your love and support. He was a wonderful man and will be missed. Open strong and say something from your heart.
Dont pry dont ask for the details. You can make a guess how they will respond to condolences based on their typical personality types but grief can make people act in opposition to how you. Offer your sympathy hug the person if it is appropriate and then back away.