Maybe remind them that cats are creatures of habit. Going out at this early stage may confuse them, especially when they realise that they are in a different garden; they may be expecting everything to be as it was at their original home.
It's always best to keep new cats in for a few weeks to create a bond between new owners, home and cats before they travel into their outside area. They need to establish the "home zone" first, and become very territorial about it, recognising it as their source of food, affection, warmth etc. Then they should be allowed into the "outer zone" where they will encounter further challenges, and be free to take to the hills and never come back. If their bond to the home zone is firmly established, they will return.
Cats become used to the challenges of their environment, including roads. If they've been used to roads and have good road sense, they tend to cope much better than cats who've been used to quieter areas or who don't have road sense. Cats and roads don't go together well at all, which is why they should try to keep these cats close to home by limiting the time they're allowed out when they do go out (hopefully in another few weeks).
You say that they are cat people who've always lived with cats. Then why do they seem to have such a casual attitude towards the cats welfare?
Hope all goes well.