A good friends daughter just committed suicide. What are the right words to use in a card?

The best words are honest and from your heart. Put yourself in her shoes and pour out your feelings. Tell her what you would want to be told.

For example:

"I cannot begin to fathom the depth of your sorrow, but I will be standing beside you through it all. I know you're not going to want to talk about this tragedy, but when you are, I'm right here. I'll always be a phone call, a text, a knock on the door, a cup of coffee away. You are in my prayers. I will hold you and your family in my heart as you endeavor to walk through this pain.

Treasure your memories, don't close your heart off to the joyful times, but change them into bright gems of remembrance. You, my dear friend, will make it through this heartache. Don't forget I'm here, and always will be."

Or something like that. Be honest with her. Assure her that you will be there but that you won't push. And do just that. Be there for her. In a couple weeks, bring her coffee and a muffin and just let her cry.

You're the kind of friend she needs right now.

I cannot give you a direct answer to your difficult time with your friend. As I've no children, nor lost anyone to Suicide. But I can tell you this, my beloved hubby/best-friend passed away unexpectedly 16 months & 15 days ago. And there was NO one who had been down this difficult path, except for one of my neighbours.
We didn't know each other all that well, as my hubby had been ill for 3 years & in that time, my neighbour was fighting Cancer. However, in our Apartment building, there is a age limit of 50+ & only 12 suites, so word spread rather fast when my Gary passed away.
His family here in Canada & mine were of no help. As I had lost my Dad when I was 16, then being an only child, took it very hard. Then my Mom 4 yrs before my hubby passed.
My only dearest relative lived & worked in Florida, and he & his dear wife were on their way up here to BC.
In waiting for their precious arrival, I've found a near urgency to keep busy all the time.
I was out getting rid of garbage, when my neighbour was returning from his morning walk.
He was the age that my Dad would have been, in addition to his similar thick English accent, was also a Christian. And very simply but tenderly, as he put his arms around me, said 'This will be the most difficult, lonely journey of your life. I've been on it for 10 yrs & still to this day, I can honestly tell you, it just won't get any better'.
It may not sound to anyone else like something good or important, but to me, just to have someone completely understand what a broken heart was really like, meant the world.
And he came to my Gary's buriel service, and let me hang on to him the entire time.
Just like my Dad would have.
In order to give comfort, you really do have to have walked in their shoes.
Sorry, as that's not what you wanted to hear, but I'm just being completely honest.

God Bless you & may HIS loving arms comfort your friend as HE has me !