I´ve lived in Mexico City for 37 years. One thing is going through a place as a tourist and another is living in a corrupt country. I very rarely get stopped on my bike but it happens sometimes. I can normally get away with whatever I have done by talking and explaining my way out of it. Once in a while that is not possible and I may get a ticket or end up giving a bribe. I always feel terrible when bribing someone but reality is that it is a way of life.
Around 20 years ago I was stopped because my plate was out of date. I explained that I had several bikes and had spent many days trying to get plates for all of them. I then said that if the cop could help me get the plate that would be great as I had run out of patience and given up. He said he had to impound the bike. I said that was fine but that I wouldn't give him any money. I thought he would let me go but we ended up at the pound. Twice on the way he asked me if I had changed my mind. I then spent 2 days getting a plate for the bike and paying fines and finally a bribe. All in all it cost me 10 times as much plus all the extra time. After that I decided that it is very difficult to beat a system that is so well set up.
In the cases where I have bribed someone I did something which I think is true in 99% of the cases. Why should you be allowed to get away with paying nothing like the case of your friend? Doesn't he feel bad about breaking the law?
I still don´t like it but it is how things are. It´s a bit like paying for a TV lincence in the UK, totally stupid but it´s the way things are.