Let me start by saying how sorry I am to hear about your friend. Death, any death, is a travesty and makes a mockery out of the life God has given us. I will pray for him and his family and also for you.
It is difficult to know what to say and do in such situations because a family’s grief is a private and personal thing and our words and actions can seem (to us) trivial and trite. Different people react to grief in different ways - some people want to talk, some people want solitude, some people may joke and laugh, other may want to be serious all the time. In many ways you need to tread carefully and read the situtation as to how to react, what to say and when to say it. It is difficult.
However there are some things that you can keep in mind.
What you do in this situation is just as important as what you say. Paul has this in mind in Rom 12:14-16 where part of sincere love is to “rejoice with those who rejoice and mourn with those who mourn”. This is practical Christian love. Simply to sit and listen to a person is more valuable than working out what to say. To make dinner or a cup of tea is a powerful witness and help.
Prayer is very important. Prayer is powerful and effective. God works through prayer even in the most difficult of situations. So what do you pray for?
Pray for healing - there is nothing wrong with praying for healing. God is powerful and can heal people miraculously if he wants. But you need to remember that he is sovereign too and if it is not his will then he won’t heal him.
Pray for your friend’s family. Ask God to comfort them and help them to mourn. Tell them that you are praying for them and you may even offer to pray with them. As non-practising catholics they ought to find this very comforting. But you can also ask God to reveal himself to them through this difficult time, that they may turn to him for help and support.
Pray for your friend (as you have been praying) that he may give his life to Christ before he dies. Ask God for opportunities to speak to him about Christ and for the words to say.
Pray about yourself - ask God to comfort you and express your feelings to Him. Ask God to help you to support them and comfort them and show his love to them.
Do you tell the family the gospel? I would suggest caution here because in the midst of grief and high emotion people do not hear the gospel message let alone understand it and genuinely accept it. It is better to live the gospel before them as I mentioned above. A time may come in the weeks ahead when a better opportunity may arise (pray for it) when the family start to seriously ask why it happened.
Seena, I hope that this is helpful for you in this difficult time. Its hard for me to know what else to say so if you have further questions then please come back and ask them.
Yours in Christ