Hi, David. I use Outlook 2013, which is pretty similar to Outlook 2010.
First, hit the Alt key; that takes you to the ribbons. Then type h,
which takes you to the Home tab. Then type c, if your intention is to
confirm acceptance of the appointment. After that, your client has
two options: To send the meeting acceptance without editing (by typing
the letter s), or to add a comment to the confirmation (by typing e
and then filling the edit box that pops up immediately thereafter.)
If your client wishes to decline a meeting invitation, following the
Alt followed by h, he or she can do this simply by tabbing to the
appropriate item in the Outlook menus and then hitting Enter once the
Decline button is reached. I'm not exactly sure how to specify a
decline upfront simply through a sequence of typed keys. But the
fields applying to confirming or declining an appointment are well
labeled for screen access in Outlook. So he or she shouldn't have a
problem after having navigated to the proper tab.