ESA whilst appealing IS decision?

My client qualifies for IS as a lone parent but has recently had her disbility premium removed due to faliure to attend a medical - she claims she did not recieve a letter telling her to attend. We will appeal this decision, however in terms of options in the meantime - I understand that she gets to keep her IS as a lone parent minus the disability premium. However, in trying to get my head round any additonal options that she may have I am confused as to whether she also has the option of claiming ESA. I have interpreted the June 2010 amendements as stating that an individual failing the PCA no longer has the option of claiming ESA whilst appealing and must claim either reduced IS or JSA, however cannot find anywhere if this would also apply to someone not entitled due to faliure to attend a medical.

I have a feeling that she will stick with the IS anyway but want to know all the options available to her before we meet.

Thanks in advance.