no, she cannot do a following-to-join for him, if their marriage happened after you were 18. That would be possible if a visa number would be available to him in a derivative category. It is not, however.
So, if the marriage took place after you were 18, only after your mother becomes an LPR, she can apply for her spouse in family based 2A category.
If the marriage happened before you turned 18, you need to file a separate petition for him.