She has done graduate work at Wheaton College.
What does the phrase “do graduate work” mean?
It’s difficult to say with no further context but I guess she worked on her dissertation.
Wheaton is an American college. Graduate work does not necessarily mean they are working on a dissertation. It simply means continued study after receiving a bachelor’s degree. They may not even be working toward a degree, although that’s the most common use. For example if someone already has a bachelor’s degree, they may only be taking a class or two. That would still be considered graduate work.
Dissertation normally refers to a doctorate thesis. The word thesis can refer to any graduate degree, including a master’s degree. It can even refer to a bachelor’s degree in some cases. However dissertation refers specifically to a doctorate degree.
Note that not all graduate degrees require a thesis.
There are actually three colleges with the name Wheaton, but the one in Massachusetts is the most prestigious and well known.