Great idea! Nice of you to be thinking of Dad! Hope my kids do when I/they are older.
First: New bikes these days are expensive.
Second: You might want to keep looking for another shop as neither of them appeared to take too much concern with their own image.
As for specific models of bikes... I don't have a recommendation. Almost every line of bikes has some sort of "cruiser" model. Make sure though, that if you do get one of these, the terrain Dad rides on is flat: these bikes are heavy.
You may want to also look into what is called a "comfort" bike. Any shop will know what that is and they can usually be set-up with a nice upright position.
Do a little bit of reconnaissance before you take Dad. See if the shop looks good, the people are smarter than the average 11th grader, and they carry what you're looking for. Also, they should be willing to fit the bike to Dad (moving saddle up/down, tilting it, tilting handlebars, etc...). This is part of the "selling" process.
Then, once you're convinced, take Dad on a day that they're not too busy with other customers (Saturday mornings will not do here). Then, you're the only ones in the shop, you can take your time, and the employees will love helping you out because they're bored.
