Hello Piotr,
Tell your brother to look for two wires cut into the lambda cicuit, usually one being grey & another purple from the lpg ecu(depending on system). Most lpg systems have the ability to wire into the lambda circuit for diagnostic purposes & does not effect the running on lpg, but can alter the signal going back to the petrol ecu, so that fuel trims don't change(by simulating a signal). If the lpg system is set up/calibrated correctly, then there is no need to connect the wires so that the signal circuit to the petrol ecu is broken, but it is sometimes usefull to tee in the purple wire to get a lambda reading on the lpg software(but more accurate viewing fuel trim data on a scan tool). I have quite often found installations where the signal wire has been spliced into, but the signal sheild wire has also been spliced at the same time. Sometimes I have found that the pins have been pulled out of the connector plug where the loom has been pulled to wire in the lpg systems wire.There could also be a fault with the feedback voltage from the lpg ecu, so in this case if the signal wire is joined back as original , it should sort it.
Hope this helps, regards, Andrew