When you call your friend to mind, you might find it helps to see them in your mind’s eye.
Imagine them smiling and happy.
You can repeat “May you be well, may you be happy, may you be free from suffering.” Or you can tell your friend what you like about them.
You can imagine giving your friend a gift.
You might want to remember a time when you were with them and you felt particularly close. Recalling this will help strengthen the feelings you have for them.
You might invite them into your creative visualization — take them scuba-diving on the Great Barrier Reef, or to a natural hot-spring in the Rockies.
You might want to think about very specific things in your friend’s life. Perhaps they’re having money worries or problems with a partner. And you can have those things in mind as you wish the friend well and even incorporate those things into the phrases you use:
“May you be free from debt.”
“May you and your partner be in harmony.”
Of course there are some things that are unlikely to change. You can say “May you be well” while cultivating metta for a friend who has a terminal disease, and you can really mean it even though you know that complete wellness is not really an option for that person.
You could however change the phrase to something you think is more appropriate, like “May you have the strength to cope with your health difficulties.” But if that all gets too clumsy you can just say “May you be well” and have a sense that what you really mean is something more like “May you be as well as possible given the circumstances.”
However I believe that being realistic is not the goal! I’ve sometimes imagined a very sick friend of mine dancing on a mountaintop, even though that’s not really an option for her. The important thing to me is that that’s what I would like for her to be able to do. Imagining this is an expression of my love for her.