I did not respond.
As Mother's Day approached I did not feel right sending her a non-heartfelt Mother's Day card and tried for 10 mins in the store to find a suitable one and gave up. 

Unfortunately that Fear, Obligation, Guilt was there when I woke up on Mother's Day.  After my lunch out with my DH and children, I called her cell #.  My choice is that it was easier for me to be superficial on the phone.  But her cell # was disconnected.  She is in her "summer" home and I called the last # I had for that.  Disconnected.

So I emailed her and wished her a Happy Mother's Day and hoped she had had a good day, and mentioned her cell and house phone numbers I had for her were disconnected.

She sent me an email back the day after Mother's Day and mentioned that she thought she had told everyone she planned to disconnect her cell # and that the home number they have had has been the same for two years (I never called that number.  I had to rescue my G'ma away from her two summers ago and we only emailed each other).  She also stated that she has listed the summer home number in previous cards to the kids (and she provided me with the #s).

She ended her very brief email with this:

"Needless to say, a mother who receives no call or card on Mother's Day feels bad and wonders what she has done to deserve such.  Mom"