You have a couple of options to ensure that your friend is insured while driving your car.
First, you can add your friend as a short term driver to your policy. Many policies include the possibility to add a short term driver at no or very small cost. You can contact your insurance and just get the name added. The exact length and amount of additional driver differs by insurer.
Second, your friend could be insured through their car insurance policy or can get a temporary car insurance policy (Zusatzpolice Kfz-Versicherung) themselves as some insurers do not offer the option to add another driver. Ask your friend to contact their insurer directly, Joonko does not offer any temporary coverage.
If you and your friend forgot about the insurance and an accident occurs, you are still insured. However, your insurance might impose fines such as increasing your deductible, reducing your no-claims class, and terminate your insurance. Your friend’s private liability insurance might cover for the additional costs but ultimately you will have to pay them first.