If your mother divorces him, she can try to negotiate for the sale of the life estate (or the transfer of his share to her) as part of that process, or try to convince the judge to grant it to her if they are unable to agree on a division of the estate.
In a court action not involving divorce, the cost would likely be considerable and, with it being an action by one spouse against another, the court may well decide against compelling the sale of the life estate. (Check the granting language to see if the life estate is assignable.) Also, appraisals of life estates are tricky, and even when appraisers put substantial values on a life estate it can be difficult or impossible to actually find a buyer at any price.

