Shaykh Abdul-Muhsin al-Abbaad was asked:
A young Muslim girl lives with her mother and sisters in the lands of disbelief where her father has left her and divorced her mother. This young girl wants to get married and a young upright man has proposed to her. So who is her wali (guardian) for the marriage? Is it correct that it be the Imam of the masjid, mufti, or a student knowledge? She is in Europe where there is no Islamic judge.
“It is well known that the Sultan [Muslim ruler] is a wali for the one who does not have a wali. If there is a Islamic judge, then the Islamic judge takes guardianship for the marriage contract of the woman who does not have a wali. If there is no Islamic judge, then the authority for the Muslims whom they return to in that land will take guardianship for her [i.e. become her wali] for her marriage contract. This is because she does not have Muslim guardians. As for the disbelievers, then they do not have guardianship over her [i.e. in regards to marriage], that is the non-Muslim relatives. This is because the disbeliever does not have Islamic guardianship over the Muslim [i.e. in marriage]”
[Sharh Sunan Tirmidhi no. 300]
Faisal Ibn Abdul Qaadir Ibn Hassan