You should say what you mean. In English, the question is not a question of idiom, but of plain, literal meaning.
The sentences would be understood either way, in their literal senses.
Now, culturally in English-speaking countries, it is usual to speak of the dead in the past tense. So, "I was his friend," would be the more usual.
A Mormon might say, "I am his wife," because that is literally what she means. Most others would probably say, "I was his wife"—although widows are granted free discretion in this. If she (being non-Mormon) says, "I am his wife," it means that she wishes to be regarded (in the context in which she is speaking) no differently than any married woman would be regarded: she does not wish to discuss her lack of a living husband. If she says, "I was his wife," then she can politely, specifically be treated as a widow.
All this is culturally conventional.
So, the meanings are quite literal; but there will be a cultural context.