I am guess that your friends are referring to 5-HTP. MDMA depletes serotonin which can cause a whole mess of problems, and 5-HTP rebuilds it back up. A lot of people like to take 400mg or so of 5-HTP when the start to come down, so they dont crash very hard. And yes, it does seem to be effective at combating the come down.
From a box of 5-htp capsules underneath a statement saying "5-htp may promote health".. "This product has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any form of disease".
What this means is that no company currently producing 5-htp can back up any claims that it's doing anything to benefit anyones health in any way.
In theory 5-htp would be converted to 5-ht (serotonin) in the brain. But that's all it is, theory.
A normally functioning brain will obviously regulate it's own production of serotonin and use it as neccessary. Anti-depressants can alter the serotonin functioning. Ecstasy will alter the serotonin functioning. But 5-htp in itself should not alter anything, it should simply be converted to 5-ht. But if the brain is self-regulating 5-ht, then the excess 5-htp would likely be expelled from the body through urine. I have never seen a documented case of anyone suffering from Serotonin Syndrome from taking 5-htp, and while it's possible that it's perhaps occured, it's undoubtedly excessively rare. If the brain self-regulates the amount of 5-ht, then perhaps it would convert 5-htp if the 5-ht was depleted do to MDMA use. I've also read reports where people have snorted 5-htp while on ecstasy and reported that it greatly magnified the effects of the MDMA, but I've never heard of anyone doing this without being under the influence.
The point is that nobody knows how it actually works. The only "research" I've ever seen about 5-htp has been based on human observation and is usually used in conjuction to sell books about 5-htp or to sell 5-htp products.
I don't doubt that 5-htp does something. But I find it impossible to believe that it magically raises serotonin to appropriate levels yet isn't a magic cure to every serotonin-related ailment.. or even a single serotonin-related ailment.
If someone takes it after using ecstasy and they think it helps, more power to them. But it may be nothing more than placebo.