Can a non-jew make coffee on shabbat using a plunger if: (a) s/he is making it solely for the jew; (b) s/he is making it for himself and a jew would like to have a cup of it.

The question is not whether the non-Jew can make the coffee – only Jews are commanded to keep shabbat- but whether the Jew is allowed to drink the coffee on shabbat that the non-Jew prepared for him. A Jew is not allowed to benefit on shabbat from a labor performed for him on shabbat by a Gentile. Even if the Gentile made it for himself and not for the Jew, the Jew cannot drink a cup because the rabbis were concerned that the Gentile might prepare additional cups for a Jew on shabbat. This if the Gentile knows the Jew. If, however, the Gentile doesn’t know the Jew at all and wasn't expecting him and the Gentile prepared the coffee for only himself and for other Gentiles, the Jew can drink from the coffee since there is no reason to believe the Gentile will prepare extra cups in the future for the Jew (Shmirat Shabbat Kehillchata I 30, 48).