The Swiss authorities online
If you live in Switzerland and borrow someone else's car, you need to consider the following:
In principle, persons residing in Switzerland should not be driving a vehicle registered abroad. If you drive such a car, it is considered to be imported and is subject to customs duties and taxation.
In principle EU residents are not entitled to drive a vehicle registered in Switzerland on EU territory. If an EU resident drives a Swiss-registered vehicle borrowed from a Swiss resident in the EU, this can already be considered as car smuggling. European Union regulations apply.
If you live abroad and enter Switzerland with a motor vehicle you can find more information here: "Entering Switzerland with a motor vehicle".