I'm 45 and a couple of companies declined to quote me on my 6 year old BMW (just a regular 2l!) when it came for renewal this year. I've got over 25 years no claims too!
He's really going to struggle to insure them. I'd kindly suggest to him that he look in to that first before he commits to buying a car that they can't actually drive.
That said, it's likely cheaper to insure them on a car that is new but has excellent safety features, than a clapped out banger that has none. DH was dreading changing his insurance when he changed cars, but they actually gave him a refund, even though he'd just bought a £40k car compared to one that was worth about £10k.
He's obviously got plenty of disposable cash, so in the scheme of things it might be a drop in the ocean.
He refers to having £300k each, so there's at least £600,000 around. If he's financially savvy he's probably getting a return of at least 3% a year on that at the moment, so £18,000 in interest ("free" money) would almost cover one of the cars. Taking a further £22k hit in to the savings that would be recouped within another year or so really isn't much of a hardship.
Is it what I would do? Probably not, because I'd be pissed off if they bumped it in a car park.
He can afford it though and obviously thinks it's a nice thing to do and the cost isn't an issue for him, so I'd let him get on with it.