Some people, to be frugal, just put water in. If he's been doing this for a few years there may only be traces of the actual cleaner left.

I've carried antifreeze before and would use an empty antifreeze jug to carry water in to make the 50/50 mix, if the jug is rinsed out a few times it would probably be safe to drive from. But now they have those pre-mixed jugs so the last one or two I've bought have been those. Might be better to go back to mixing it myself at least I'd have extra water if needed. I helped a lady and her son in a rest area, the boy was running a paper cup up to the water fountain and running back and pouring in the radiator but I think it was running out as fast as he could fill it. There was a tiny line, one of the little hoses that circulates water around the intake for emissions that was rotten and leaking. I had a scrap piece of fuel line in my toolbox that fit so I replaced it and gave them my containers to refill.