This is so stupid. I can't believe people are so focused on such a minor change. Seriously this isn't the end of the world. All that time spent fussing about this button could have been put to better use elsewhere.
Usability first.
In addition to her own confusion, add mine when I'm telling her on the phone to look for the "plus" icon at the right, and it's no longer there.
I've had similar issues with Google's Calendar product UI, for example. They just change it, and adopt non-standard and to my mind often rather unobvious controls -- users be damned.
(I won't start on my parents' use of Android phones...)
TL;DR: You want market share, you'd better make sure that market can figure you out.