1. Does a baalas teshuva, whose biological father is a goy, have an issur yichud with her father?
2. If not, then if she is with him, is she a valid shomer for him with one other Jewish woman?
2) One may be lenient where necessary.
Sources: There is no yichud concerning a biological father, even if he is not Jewish. Concerning the question of acting as a shomer, Shulchan Aruch rules that a non-Jew’s wife is not considered a shomer, but this would appear to be a special halachah in his non-Jewish wife. Concerning a Jewish (Orthodox) daughter, she would be considered a shomer. However, there is a machlokes (even for Jews) over whether a daughter is considered a shomer or not. There is room to follow the lenient opinions, where necessary.
Concerning a shomer for a non-Jew, see Chavas Yair 66; Chazon Ish, Even Ha’ezer 35:1; Devar Halachah 7:14 (see also Shulchan Aruch 153:4); concerning the question of a daughter as a shomer, see Yosef Ometz 26; Pischei Teshuvah in the name of Zechor Le’Avraham; Mibeis Levi, vol. 4, p. 48 (permitting).