It will become a never ending cycle - something that continually drives the need for animal control, btw. The city can only deal with it on a temporary basis, and if they issue a ticket or something else that hurts the owners it not only doesn't "fix" the problem but you might have a price to pay as well. There are thousands upon thousands of complaints out there, and AC in most cities ( Calgary is one big exception) doesn't really do much for the long term except for some very specific types of calls. (If these were the kind of folks concerned about tickets the cats *probably* wouldn't be intact, in a very general sense).
But your question is about how to deal with the conversation - the TNR group sounds good, and you might also check with Best Friends - Phone (405) 418.8511, see if they have any advice or resourcs that might help. I don't know how much help they might be with this, but it is worth a shot. I used to work with them a bit years ago.
Your best bet is to help them get this done at the ABC clinic, and return the cats to their place, help them rehome the kittens.
The ABC clinic is a great asset, however, especially if you can help them with transportation. It's the cost that is the greatest barrier (among others) with cat owners, among others.
By being a patient friend and resource you might be the best chance they have.
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