A couple of years ago, I had a friend who had not ridden in a long time. I told him to go out and look around at everything that he liked as far as looks go. And then, sit on them. To see what felt right for him, what felt comfortable, and natural for him. He did, and found one that was right for him at the time. He loved it, eventually trading up for something bigger, more powerful.
So I'd have him go out and see what feels right to him. As far as Triumphs go, the Bonneville line has many different models, and would be a great bike to really get to learn on, all would be very easy to manage, and have enough power not to get bored with.
Pat.