We have 4 motorbikes. I don't ride, but love riding pillion. I grew up with bikes, I was 4 the first time I rode on the back of one.
DP has been riding bikes for 41 years, including 13 years as a despatch rider in London, clocking up huge mileages. He had a few spills in the first year or two, and then a car hit him when the driver changed lanes without indicating and dislocated DP's hip (he's adamant that he would have been fine if he'd had a modern bike, as their brakes are so much better). But he has been accident-free for 38 years. He is the safest rider I have ever met and seems to have developed a spooky sixth sense, he seems to know if a driver hasn't spotted him and is going to pull out of a side road.
For me, there is nothing like the sheer exhilaration of tanking along a country road with long sweeping bends - it's fantastic fun.
However, if your DP hasn't ridden for a long time, I'd suggest that he gets a modest bike to start with until he gets his "bike sense" back. So many "born again bikers" go and get some superfast sports bike not realising how much faster they are than the bikes they rode years ago when they first learned, and I believe middle-aged men who have recently returned to biking after many years in a car.
And proper gear is vital. We wear kevlar jeans or leather trousers, we both have jackets with shoulder, elbow and back protection, proper tough boots, gloves with stout protection for fingers and top of the range crash helmets. You can even get leather suits with air bags built in that protect you if you crash.
If he really loves biking, and you can afford it (all that gear doesn't come cheap, and bikes aren't a cheap means of transport like they were when I was young, there's insurance, running costs etc to consider too) I think it would be mean to stop him.
Get a bike that's nice and comfy for a passenger and you might even get the bug yourself. We sometimes use the touring bike to go shopping, because it cuts through the traffic, can be parked almost anywhere and a week's shopping fits easily in the humungous panniers.
And if you want a taste of just how exciting, MotoGP is on BTSport2 all weekend. Thrills and spills, and you'll see how just how safe all the right gear can be, but bear in mind that these guys are the best few riders in the world and can practically defy gravity when it comes to ludicrous lean angles and getting the bike upright again.
This thread has got me all excited. It'll soon be warm enough for me to want to get out there again.