Unless you've completed technical diving training covering oxygen toxicity risks and safe gas switching procedures it's really not a good idea to carry an oxygen stage or pony bottle when diving. You don't need it for recreational diving, and it introduces additional hazards and equipment complications. When I do recreational shore dives, I keep an oxygen stage bottle in my car. The odds of actually needing it are extremely low, and if you do need it then taking a few extra minutes to swim to shore is unlikely to make a difference. If you're really worried about the time factor, then I would suggest using a DPV so you can get back to shore faster instead of trying to haul the oxygen around in the water.
They are Cmas *** divers, so already used and certified for deco dives and max depth 50m.
Although in Cmas this is still fully recreational, in other organizations this is considered highly technical.
They also pman to get Advanced Nitrox, which covers accelerated deco with high oxygen mixtures up to 100%...
So yes, albeit recreational, they will be fully prepared for using a deco tank with 100% oxygen.
It is the idea of using that tank as an emergency oxygen supply which is weird.
A distressed diver with DCS symptoms is unlikely to be able to breath from a regulator outside water. You need an oxygen mask.
On another side, I do not think it is a good idea that a couple of divers do a shore diving leaving no one on shore waiting for them.
I also started in a diving club. And when I and my girlfriend had enough of such "group organised diving", we did simply start diving with a couple of our friends from the club.
In 4 you can easily afford to rent a boat, sharing the cost. We did usually dive two first, with the other two on the boat, and thereafter swap roles.
2 in water and 2 on boat is very safe. And you can carry on boat a full safety kit.
And even when shore diving, we did the same: 2 in the water, 2 on shore. And thereafter we swap.