How soon is too soon?

My mother passed away last year, last march to be exact…it has been a little more over a year. My dad and i were devastated by the dead of my mom…but we were able to get on with our lives, recently he has been keeping busy by attending some free computer classes at the public library. He is 72 years old, is a great dad and was a loving husband while my mother was alive, he took great care of her till her last breath, but lately he has been bringing lots of new friends home…men and women…which i have no problem because i believe socializing is always good at any age. But today he invited in particular a lady who is divorced over to have dinner plus another couple of his friends; after my mother died i moved in back with him to keep him company… so today while i was heading out..i caught them kissing….i just kept walking and ignore it..but i have no idea what to do… is he over the death of my mother that quick?...is he looking for a female companion to forget the pain that was losing my mom?..or am i just looking this the wrong way…please any advise would help. Thanks.