I love pay toilets. Most of them are kept clean, and I don't consider finding a clean, available toilet to be a right.
I've paid for others to use toilets, and I'll keep doing so.
"Don't let people pay for things because poor people might not be able to afford it" is offensive to "poor" people.
Oh well. If it makes him feel like he's championing a cause...
Posit a reason why it is socially unacceptable for people to just crap in the street, then.
(This is the difference between a negative and positive right)
If someone has a right to clean, available toilets at all times, then someone must pay to provide those services. I'm happy to donate to the cause, but I don't think the force of law should compel me to part with my money to buy someone a toilet.
Especially because PAY toilets solve this problem so elegantly.
For example, imagine a pay toilet that, when you pay, you have the option to buy access to the same toilet to someone who is unable to pay themselves. I'd do it. This is basically the same idea as the "leave a little, take a little" coin jar many shop vendors have.
If you need 20 cents for some reason, take it. I usually just dump my extra change into that jar. I would do the same in a pay toilet, and often have.
It's important to distinguish what kind of "things" you're talking about, though. There's a pretty big moral difference between not being able to afford consumer goods and not being able to find an appropriate place to perform basic bodily functions.