The expression "low will power" is negatively charged. Using it might cause a person with an eating disorder to feel guilty. He or she won't benefit from feeling guilty. It doesn't improve the condition in any way and only produces feelings of failure and worthlessness.
However, if an overweight person is going to get thinner, then a diet with a low amount of fat must be used. If the person concerned is fond of a fat diet, the change will, of course, require considerable personal effort.