I have a LOT of experience with taking both MDMA and LSD (and other psychedelics) in therapy, at raves, out in nature, etc. I've had many great spiritual insights. I tripped with Timothy Leary---wow--so many great memories!
In "The Psychedelic Experience", Leary, Alpert, and Metzner write that there basically are no "bad trips". What I mean by that is that IF negative emotions should come up during a trip, it's because of some unresolved issue that is emerging under the influence and this issue really needs to be worked though. Work through the issue, and your friend can go right back to having a "happy" trip.
It can sometimes be more difficult to work through negative emotions in public, but sometimes not. I've done lots of tripping in public, at rock concerts, etc. I always make sure to do it with a very good friend, though. That way, if I should momentarily experience a negative emotion, my friend is there to soothe me and I'm not all alone. I'm a pretty happy tripper, though. My friend told me he'd get paranoid tripping out in public the way I do, but I never get paranoid---just happy. I do like to combine MDMA and LSD, but that's just me.
Just about the only time that I had a very negative reaction to psychedelics out in public was when I was at a party on LSD. I was doing GREAT on the LSD, feeling so HAPPY. Then a friend of mine who is a pothead insisted on passing the pot pipe around. I take full responsibility for my own actions---I should have said "NO" to marijuana. For whatever reason, pot is one of the few drugs that I never seem to have a positive reaction to. As soon as I smoked the pot, I went from feeling total BLISS to feeling like I was having a panic attack and couldn't breathe! I also felt very paranoid, because pot often makes me feel paranoid. Another friend had to take me outside, away from the people at the party. We sat out on the steps while I came down from the pot. Once I was back to being just on LSD, I was a happy tripper once again.
Any negative patches in an MDMA or LSD trip can often be worked through with the support of friends. Some people learn the most from the negative patches. I'm usually a positive tripper, but I think that some really weird music might affect me negatively, too. I'd probably be feeling like I had to get away from that kind of music. If there's an outside balcony or something, I'd go out there. I agree that a change of scenery can do wonders. I believe this is the reason why most raves have "chill out" rooms with soothing music, at least most of the raves that I've been to. Chill out rooms are an excellent idea to help get a tripper back on the happy path to bliss.