Ah Gelda and Ruthann, ladies after my own heart...my first thought was greyhounds too!
whisky, retired greyhounds are lovely laidback dogs who only need a couple of walks a day (which would be great for your dad too!), some food and a little love. They are beautiful animals. They usually cope well with being left for a few hours too, although they would need to get used to their new routine like any other dogs. Also, some greyhounds aren't good around smaller furries, so if your dad has a C-A-T for example, or neighbouring houses do and he plans to let him outside, he'd have to be careful...
Also, rescue centres are crying out for peopl elike your dad who can offer a lot of time for a dog. There are lots of older dogs in rescue who've been in and out of homes and I think they deserve a new start in life. As jules pointed out, with your dad being in his 70s, an older dog would give him companionship but wouldn't necessarily live for 15+ years like a pup could. Rescues work really hard to find the right people for their dogs, usually, so your dad's new friend would be very suited to his lifestyle etc.
Lots of luck, whatever his choice.