Could be that the person you met a) was purposefully misleading you, b) was something other than LDS and you misheard them, c) was LDS and was yanking your chain, d) was LDS and there was a misunderstanding, or e) was LDS and was a nut job.

I've been an active member all my life. Only "prophet of" reference I've ever heard is "prophet of God." A prophet is one who bears special witness of Christ. Therefore, a "prophet of Abraham" would be antithetical to the LDS (or most of the Christian world's) understanding of what a prophet is.

As for the star issue, the only thing I can come up with is the apocryphal "signet of Melchizedek," or "Seal of Melchizedek," which is not an LDS doctrine, but rather an image of historical debate. The signet is an 8-point star that some throughout history have claimed to be the symbol of Melchizedek's priesthood. This is, again, not part of any official LDS doctrine, but has been researched by some LDS scholars. That's about as close as I can come.