Shock absorber mix at 30’s h&r springs ring srum

Good evening. I’m the new one here and I have a direct question I sold my Opel signum the penultimate weekend privately. Now the new owner writes to me that this is not correct with the chassis and therefore I am supposed to replenish myself or to participate in the funnel of the “error”. The legal situation aside. I’m more interested…is it permissible to mix front serial shock absorbers with rear FK shock absorbers. As springs are 30s H&R ru ndum. He has an expert report on them, but says that his workshop tells him that this is not possible, it would be forbidden. Is that so or is it not so? Rejoice me (hopefully) to your answers 😛