workers comp and cobra

in the state of ny. my sister was on workers comp from april to august 2000. she works for sears and recently receieved a letter from ushealthcare that her coverage will terminated effective may 31 2001 because she failed to work the mandatory 1000 hours required to maintain health coverage under the group plan. the reason she fell short of the 1000 hours is due to the time she was on workers comp. are they correct in forcing the very unaffordable cobra and terminating her from the group coverage she has with sears?