In addition to the other good advice already mentioned, I just wanted to add a few things. Firstly, because you never know the mg dose in any MD** pill, you can't apply the LD5o rule - you just don't have the info needed.
The next difficulty with your question is that some people respond differently to MD** than others. For example, some have pre-existing health problems they may not be aware of (like a heart problem) so taking a large amount of MD** is more dnagerous for them than for others. Then, different people break down MD** at different rates because of different quantities of the enzymes that do this job. Again, this puts people at more risk than others.
Then there is environmental and behavioural factors that contribute to health problems like high temperatures in a venue contributing to hyperthermia. Dancing for prolonged periods can lead to muscle breakdown, etc, etc
Finally, if people take certain medications it can effect the metabolism of MD**, again leading to problems.
If my friend took 13 pills, once he had recovered I would sit him down and tell him if he ever did it again, I would never go out with him again. I would say it ruins my night stressing that my friend is gonna keel over and die. Suggest he set a limit on how many he will take in advance and that you are happy to party with him if he sticks to it. Encourage him to reflect on how the night felt, and the comedown - was it worth it (I doubt it). You can do this in a supportive way, stressing it is out of concern for him that you are doing it. Good luck with it