The situation is :

My friend's father has died. My friend is very sad. I am sad too.

I want to express my sympathy to my friend.

I want to condole to my friend.

How could i do that?

There are many things to say, but a simple thing is 'I am very, very sorry about your father'.

Best wishes, Clive
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A traditional phrase in English is, "I'm very sorry for your loss. My condolences."

A similar construction is "My condolences on the loss of your father" or "My condolences on your father's passing."

When it is someone I know very well, I sign my note,

"Sharing your grief,
-- Ankhorite"

Other people might say, "sharing your sorrow" or just "sincerely" or "with love."
thank you very much sir.