Your garage is right, although “never” is a wide understanding.
Reason why it can go wrong, is that the moment the car, the one which is jumpstarted, fires up: there usually develops a peak current. This peak can destroy various sensitive control units.
Personally I have experience with my previous car. That was a 156 1.8. During a very cold winter period and tired battery as a reult after weeks and I didn’t even jump start it. But on one very cold morning and just only on its very deepest breath while starting, it fired up eventually. Glad it did, but immediately after that the ABS CU was blown (fault code could not be deleted).
If you need to jumpstart, best to put the leads widely spread from each other; i.e use the positive terminal of the battery and clamp de negative directly to the motor. And even than, I would first have it charged for a couple of minutes to restore some of the battery.
So for modern day cars: be careful!
Last edited by Leo_NL; 25-01-19 at 19:00.