What are the effects of a reverse-connected battery?

If something can be done, eventually someone will definitely do it ...
When replacing the battery, an amateur wanted the help of a friend and when the car still did not want to start, he said that maybe he could try to change the poles .. and it happened! Specifically, it is about Peugeot 106 from 96. Has anyone encountered such a situation and could say what damage could such a connection cause? Today the car has been towed to the mechanic and I don't know what to expect.
Burn the radio, although often in their structures I saw diodes short-circuiting the power supply with reverse polarity, which caused the fuse to blow earlier. But what about the rest of the electronics? There is no ABS, no lock in the above-mentioned one; factory code lock only.
I am asking for opinions.

You can expect a smile on the mechanic's face ;-)

And when it comes to damage, it depends on what was done.If only the switched battery, the losses are definitely smaller than after trying to start the car, maybe the manufacturer predicted that someone so capable will be found and used a similar protection as in radios, it is difficult to say without seeing the equipment. Such situations rarely happen.

PS
How did someone manage to press the clamps on the battery bolts in the opposite direction, after all, they have different diameters.

there is still a high probability of damaging the ECU, the dashboard if it has some electronics and other electronic elements in more technically advanced cars

Added after 11 [minutes]:

I will add one interesting thing that happened to me a few years ago. namely, a friend of mine charged the battery so that where it should be a plus, there was a minus. it was then 126P. with the alternator, the only fault that occurred at that time was the lack of an indication of the fuel level. the reason for the burnout of the feedback diode. he changed the clock 3 times, nothing helped. only when after 2 times of elution of the djode by me and its immediate burnout, I took a closer look at this case and it turned out that the battery changed its polarity, while the car was running normally.

Hello.
There is a likelihood of damage to the ECU as already mentioned above as well as serious damage to the fuse box. For this, most likely, the electronic immobilizer could have caused the vehicle to be blocked.
By the way, applause for the "helper".

After the repair, he reports:
- blown diodes in the alternator (the same effect as in the only post found on the electrode describing such a situation, it was the case of the Audi 80)
- a few fuses came
And that's it. Costs, after the fair, PLN 90. In fact, probably lucky that the ECU survived.
Thanks for the response.

I connected the reverse to the Volvo and what was the effect, he did not want to start, he only shot in the other side hahaha, but after turning the battery over, he only fired without charging. A ground wire at the alternator is burned out (the alternator has no ground connection through the housing). ECU damage is of course possible

In itam overwhelmingly sits down alternate specifically, the diodes in the alternator but in Japanese the alternator is overwhelmingly secured yours