iCal invitation, no option to accept, decline or tentative

We are a small office running OSX Mountain Lion Server. iCal invitation to local user does not give the option to accept, decline or tentatively respond. invitee is running Yosemite on Mac Pro. We tried re-installing invitees iCal account with no luck. iCal events received by email will import into iCal when clicked but only give the user the option to select "ok" from the event inbox at top right. Why is it not showing the accept, decline etc. like every other user??

thank you,