The 1098 and 1198 are very powerful. The S2R is a totally different ballpark with a Desmodue, whereas the 1198 is a superbike motor. You're literally going to be going to a night and day difference with horsepower. Not to mention the ergonomic style of the bike is entirely different from the Monster S2R.
I would say if your dad enjoys going around town and in the twisties on the backroads to stick with the S2R (I'm being honest) and if he wants a superbike that is lightweight in comparison to the S2R i'd go with an 848 instead. The 1198 and 1098 are awesome, and fun, no doubt. But as far as daily use, they're insanely overkill for every day riding, backroad twisties, etc.
Now if he plans on doing trackdays, i would say GO FOR IT. You'll see it's entire lively attitude out there on the track. But on the street, you'll never utilize the full potential of the bike. The S2R is also geared much shorter than the 1098/1198 so it's more fun for small rides, backroad twisties and is more comfortable. I found the superbikes to be less fun on super long trips. They're more fun on trackdays, and far more fun if you have a wide open road and lots of corners, but in comparison for every day riding... S2R hands down.
This is just an opinion. I've ridden a lot of Ducati motorcycles. All the way from their sport touring bikes, superbikes, even the multistrada, supersport series and the older superbikes. Out of all of them, the 848 and 1098 and 1198 are all very awesome bikes. But they all serve a purpose. So I would say it would be a lot better for him to ask himself "What do I plan on using it for?" if he says touring... I'd say go with the S2R. If he is all about sporty riding, it doesn't bother him to be on a bike like that in traffic and it isn't a hassle to him, then go with the 1198. They're both superb bikes. But like I said... they both serve an entirely different purpose and riding style.
On the flip side you need to consider that the superbikes are a much higher maintenance and much more expensive to get parts for. So he will pay more to own an 1198 than he will that S2R. The 999 was dubbed the highest maintenance ducati they ever made... the 1098 changed that and made it much better, so less mainteannce, but superbike is a superbike. So the maintenance is more and the parts are more, that is the trade off for more performance.
If I had to sit the 848, the SuperSport, the S2R and a 1098/1198 side by side in a garage.... in order, i'd put the S2R at the far left for comfort, fun, and use. The SS would be just past that, little more sporty, little more power, less comfort, not really great in town. The 848 is wonderful in the middle, sporty, lightweight, handles great, still fast and quick, not insanely comfy for long rides, but a lot of fun. At the far end of the spectrum you have the 1098 which is very fast, handles great, little bigger, little heavier, not super comfy for long rides, but does the job and does it well for sporty. Hope this helps.
Remember - every bike is different
Whichever route he takes he won't be unhappy, it's still a Ducati!
have an awesome weekend.