Well this one is tricky! I am a conservator and would recommend that you consult a conservator in your area if possible. The acetone would likely alter the luster of the hair and could lead to further embrittlement. A conservator’s approach might be to use a gel which would hold the solvents in suspension while the adhesive was softened to the point of being able to be gently reduced. However, the structural integrity of the horsehair and the solubility of the adhesive will ultimately guide the treatment. Good Luck!
