The costs have dropped to zero for many of the smaller low traffic sites because they don't currently need more resources than those provided by Herkou's basic free service. We can boost the resources almost instantly as and when they need it.

You can definitely host tiny apps for free.

If they are fairly low traffic sites with small databases (< 5 MB) that don't require SSL or more than a daily cron, they should fit within the parameters of a free plan - i.e. "blossom" database server, 1 dyno, etc.

He could be using Heroku's free accounts.