What are the current criteria for the Tourist Visa Exemption for German passport holders visiting Thailand?

Immigration would rarely ask for any of these unless they have a long history of previous entries and the immigration officer is looking for ways to deny entry. They could ask but rarely would.

The issue may be with their airline. Some may insist on a flight out within the timeframe of their entry allowance. Some airlines won’t.

If you want to cover all bases then as

@El***

said. For accommodation proof email them a letter inviting them to stay at your house for the period of their stay. Very unlikely they’d be asked, but they’d have it on the rare chance they were asked.

You could write a simple letter of invite to stay with you containing their details plus your address full details.

In case they are asked for, they need to show:

1. Proof of outbound flight within 30 day

2. 20K baht per person or 40K baht per family or equivalent in cash

3. Proof of accommodation

they can use their facility with proof (contract etc).

Unless they are frequent travellers to Thailand, it's a low possibility to be asked for these.