Why my friend weigh 80kg at 186cm and look lean skinny and doesn't have muscle mass? is it because his bones are heavy?

Your friend is tall (his BMI is 23.1) so he does not need to be muscular or has heavier skeleton to have this weight.

Never blame weight on heavy bones. You did not say what his other relatives look like. I would compare them and see if it runs in his family. Discuss with him. He may want to do a skinniness family tree if he is worried about it.