Hi. I`m Mexican and live in Mexico city. 50 pesos is a huge tip...In restaurants you usually tip 15% of the total of your meal. For people at the hotel, I would say something between 20-50 pesos. 50 pesos is a LOT. I think I have never tipped that much in my life. All this applies to the city. If you go to these really expensive places (Can-cun, huatulco), then you probably need to give a bigger tip. I would think 50 pesos would do anywhere. At the airport, my parents always tip 10 pesos for each piece of baggage.
I hope this helps. Where are they going? I`ll be happy to help with anything they might need, or any questions they might have. I can pm you my phone in case they need something when they are here. It`s always good to have a contact number in case something happens.