He assured her she would get the opportunity to be sealed to our daughter in the afterlife and she wouldn't need me or any other man for it.

Unfortunately, your bishop taught false doctrine. As our dear profit brigham taught (and remember the profit can never lead you astray), if a woman is married to an unworthy husband she will be given to a worthy priesthood holder in the celestial kingdom as a sister wife. Your child will also be sealed to that man. Heck, she might even be given to your bishop or even brigham himself! That way she can get the blessings of polygamy and eternal motherhood without being penalized for being married to dead weight.