I understand that a Jew who converts to mainstream Christianity, believing Jesus is God, is considered according to halacha as a Jew who cannot be counted in a minyan, or buried in a Jewish cemetery, etc. However what does halacha states with respect to a Jew who happened to believe Jesus was merely a man who was messiah, but not God, or divine, etc.. Would they be also considered as a Jew who cannot be counted in a minyan, etc.?
Let me begin by stating the obvious - Jesus WAS merely a man who was certainly NOT the messiah! One need only take a cursory glance at the Tanach to see what the messiah will achieve (peace, rebuilding the Temple, closeness to Hashem etc), and a cursory glance at the world today, to see that we still are awaiting his arrival.
Having said that, it should be clear that someone who holds such beliefs is very very mistaken. I would suggest the following reading list - "The Real Messiah," by Aryeh Kaplan, "Faith Strengthened" by Isaac Troki, "You Take Jesus, I'll Take God"and "Their Hollow Inheritance" by Michoel Drazin (available at Jewish bookstores everywhere). I hope you have success helping this person reach a true understanding of Torah. (The following link to the Ohr Somayach website will also help - http://ohr.edu/ask/ask00j.htm#Q1.B).
As to your specific question - if there ever was such a person, who believed totally that Jesus had no divine aspects, that much of what he is recorded as having preached is totally wrong (i.e. that the mitzvot no longer apply, or that Jewish people no longer have a covenant with G-d), and that Torah as we understand it through the Oral Law is still totally valid, and will never change or be added to - if there was ever such a person (which I highly doubt), then I am inclined to believe they are still valid (though troubled and mistaken) members of the Jewish community, and count for a minyan.
In the responsa B'nei Banim (Rav Y.H. Henkin, shlita"h, vol 4, 26) he writes of someone who believes that Rabbi So-and-so is the messiah, and asks the same question you ask, as to whether they are a heretic. He answers that though their belief may be totally wrong, and quite absurd, the believer is not a heretic. For in the Gemara we find Rebi Hillel who believed that the Messiah would never come, which goes against basic Jewish beliefs, and yet he was not branded a heretic - merely wrong (San 98b). So too in this case - a very wrong belief, but not heretical.
Again, I stress that it seems unlikely that such a person does not hold other heretical (and not just mistaken) ideas.
May we be blessed to see the truth of Hashem's wonderful Torah spread and be seen by all.