Gosh if a granny can't drop everything for the sake of a poorly grandchild, what hope is there. Presuming your friend is not in the latter stages of a debilitating illness, recently bereaved and not coping, or truly desperate for time with someone/anyone, then family, especially in an emergency wins hands down every time. No need to feel one iota of guilt, taking time off from teaching is very hard (ironic in what is meant to be a child centred profession) so my instincts would be yours- within the bounds of what is possible, and sadly at the moment, DH's health trumps even poorly DGC 
Even so DH was prepared to try to muddle through the other week when DD was down with tonsillitis and really incapable of even picking the DGC up from nursery and school. Fortunately, SIL is not a teacher and was able to work from home so that he could take and fetch, so I wasn't needed.
Do not feel guilty, you are excused for feeling let down by a friend who is taking the huff. Sad, but he'll get over it!