Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "When a woman's guardian is harmful and the woman appoints another person as her guardian, and he conducts her marriage, then the marriage conducted by him will be valid."
Hadith Referenceسنن الدارقطني / كتاب النكاح / 3884
Hadith Takhrij«أخرجه الدارقطني فى ((سننه)) برقم:3884 ، انفرد به المصنف من هذا الطريق»