According to the Swiss government's Information on Declaration of Sponsorship (pdf), a guarantor cannot legally vouch for a guest's departure from Switzerland. Any statements you may have made purporting to guarantee your guest's departure from the Schengen area are probably therefore without effect.
If you submitted such a form, you have undertaken to guarantee the payment of certain costs related to your guest, should they arise:
By signing the declaration of sponsorship, the guarantor undertakes to cover the following costs:
Costs arising from sickness, accident, return transport and living costs, which would otherwise arise for public welfare or private medical services during the applicant’s stay in Switzerland
A maximum of CHF 30,000 for individuals or groups and families of up to 10 people who are travelling together.
Since the document appears to cover costs arising from the applicant's stay in Switzerland, as opposed to the Schengen area, you may even be off the hook if your (former) guest incurs any such costs in Germany.
Without having access to the actual text of the form, it is difficult to say anything more precise. If you can post the text of the guarantee, perhaps as an image of the form with identifying information blacked out, it may be possible to analyze it in more detail.
As you indicate in a comment that you didn't submit a formal guarantee but rather an informal statement in your invitation letter, the government probably discounted the guarantee from the start. Such guarantees generally do not carry any weight, since a sponsor or host is not really in a position to offer the guarantee.
In most cases, a guarantee says that the guarantor will do something specific if a certain condition is met. For example, when a merchant offers a money-back guarantee of the consumer's satisfaction, the merchant is undertaking to refund money when the consumer is unsatisfied. The guarantee you gave has no such backing: you haven't undertaken to do anything in particular if the person does not leave.
You are perhaps right to wonder whether your credibility would be called into question for future visa applications where you are the sponsor. There's not much you can do about that at this point, except to hope that the Swiss don't notice. It might also be a good idea not to make any claims about your prospective guests' plans to leave the Schengen area; that's normally a matter between the state and the traveler (unless the host has signed the formal declaration of sponsorship, of course).
If the host has not signed the declaration, then the host's role in the visa application is generally just to confirm the traveler's itinerary and that the traveler has a place to sleep. The host does not play a significant role in the assessment of whether the applicant will leave the country.