It is, but it could be taken to mean "he considered me as his friend" where he's still trying to decide or is making a decision and is taking how he feels about you into account. For example, if you asked him to keep something critical secret, you could narrate his thoughts as "he considered me as his friend."
I wonder if the general sense of this statement is the same in English and Spanish. In English, a likely interpretation of this sentence, (He considered me his friend.), is that he no longer considers me to be his friend.
The reason for this is that the simple past of English often (but not always) signals an event that has completed in the past and is no longer impacting the present. English has the present perfect tense (has considered) for things that started in the past and continue in the present. [ I know that American English often uses the simple past in place of the present perfect.]
My question is, does the given Spanish sentence, (Él me consideró su amigo.), suggest that he no longer considers me to be his friend?
I find it easier to understand the difference between the preterite and the imperfect if I think of how they are used in storytelling.
The preterite describes something that happened which is complete in the sense that the speaker is done with it and ready to move to what happened next. A sequence of preterite verbs describe events in the order that they happened (absent time expressions to the contrary). These are not instants of time, but they do have specific durations--even if the speaker doesn't say how long.
The imperfect describes something that requires further explanation. A sequence of imperfect verbs describe the setting for something and leave the listener expecting to eventually hear a preterite verb that will advance the action.
If you say Me consideraba su amigo, then the listener takes that as a description of the environment in which the story will take place.
If you say Me consideró su amigo then the listener expects to hear about something that happened after that time. Somehow you lost touch. Or the other guy died. The next sentence or two should say.
imperfect would cover that - as in 'he used to consider me his friend' - preterite seems strange to me as a french speaker as unless he literally 'considered' for one moment in time, it would be imperfect as it describes an action that was ongoing in the past....anyone like to explain the spanish way of thinking about this to me?
Thank you but I still do not really understand what is the difference between these two sentences. Does "He considered me his friend." means that he is thinking if I should become his friend and "He considered me to be his friend" means that he thought that I am his friend but I am not?
Those phrases do have subtle differences. Your phrase would be a translation of "él me ha considerado un amigo." If you add the word "has", it implies that, while he did consider you his friend, it was only from time to time, or that it was short lived.
You will find that in Spanish they employ even more of these subtleties than in English. Ask a Spanish speaker what the difference in these two phrases is "Podrás hablar español?" and "Puedes hablar espanol?" Or better yet, try to research it yourself.