ich wünsche dir viel kraft
die mutter einer brieffreundin ist an krebs verstorben. ich möchte ihr (also der freundin) viel kraft für die nächste zeit wünschen. wie mache ich das?

Ich wünsche Dir viel Kraft.
Die Mutter einer Brieffreundin ist an krebs gestorben. Ich möchte ihr (also der Freundin) viel Kraft für die nächste Zeit wünschen. Wie mache ich das?

I wish you a lot of power for the next time.

not ..."for the next time", that reads as if you're expecting someone else to die of cancer soon.

I'd go for stregth, not power.

I go with Tanja1 Stength.:)
strength to overcome the pain of losing someone you love.

neither fortitude nor power fits, and especially not "for the next time"

I read once: "strong birds will survive a broken wing and live to fly another day"

Which is quite nice.

You could say something along the following lines:

I hope you and your family find the strength to get you through the hard days ahead. You will be in my thoughts.

There are lots of other ways of expressing your sympathy - I'm sure other people might have appropriate suggestions.

That's life. Be strong. Grieve. Get over it. Get on with your life. Good luck.

>I hope you and your family find the strength to get you through the hard days ahead. You will be in my thoughts<

Use Jenny's suggestion.