.243 is a great cartridge, sounds like just what he needs. If you want to shoot 100 gr bullets I'd play it safe and get a 1:9 twist. 1:10 will be a bit close to not being able to fully stabilize a 100 gr, but it might. If you will be shooting 85-95 grain, the 1:10 will be fine.
The only beef I have with the Ruger is the stock. It's very cheap and flimsy and you're going to have to remove some extra material in the barrel channel for it to be truly floated. Oh yea and forget about using a bipod with it. Also nobody makes an aftermarket stock for it so you're stuck with what Ruger gives you. Other than that its great.
The only beef I have with the X7 is...well, the stock. I don't like the idea of it needing a pressure point for the barrel. Boyd's makes some nice wood stocks for the X7 that float the barrel. But about half the time they shoot as good or better, half the time they shoot worse after floating.
They are both good rifles for the price. The new Remington 783 seems like a decent rifle at the same price point but I haven't been around one so I can't give any guidance there.
For just a few dollars more you can have a Howa 1500 or T/C Venture, which is a step up from the three previously mentioned rifles IMO.
And for a few more dollars you're in the Savage (Axis line doesn't count as a real Savage IMO), Weatherby Vanguard, and Tikka T3 range.
My advice is to take that $350 you were going to spend on the RAR or X7, save another couple hundred and get something of much higher quality that you and your father both will be proud of for years to come.
I believe ~$550 will buy you a new Tikka T3 lite like this one
Also don't forget there's always the used market. Lots of .243's out there