Sandra, who I work out with, will not be a member of my gym anymore. (whom).
- Sandra, whom I work out, will not be a member of my gym anymore.
- Sandra, with whom I work out, will not be a member of my gym anymore.
- Sandra, whom with I work out, will not be a member of my gym anymore.
- Sandra, with who I work out, will not be a member of my gym anymore.