Your guest may enter without a visa — so the formal obligation is off the table. No office appointment, no fee, no liability stretching five years.
What remains is smaller, but it is not nothing.
The three things that stay
Evidence of purpose and means. Article 6 still requires both, only not in the shape of a form. In practice a return ticket, the address of the accommodation and sufficient means will do.
Insurance — optional, and the most expensive thing to skip. It is not a condition for visa-free entry. Without it, your guest pays for any treatment out of pocket.
Counting the days. 90 days in any 180, rolling, and across all Schengen states together.
What you should NOT sign
Some families obtain a formal obligation anyway, “just in case”. That is usually a poor trade: it costs €29, is accepted for only half a year — and creates liability lasting five years. For a three-week visit with no visa procedure it almost never pays off.