Two pair (I'm guessing he has them paired) of large, aggressive fish in a 90g is marginal at best. THREE pair, with one being jags, is never going to work! He should get the jags out and cross his fingers on the other two pair. As posted above, the jds are the LEAST of his problems!
I have one gt and one ebjd (with some other milder cichlids) in a 75g (which is the same footprint as the 90g - depth doesn't really matter much with cichlids), and I'm concerned about what will happen when they get to be 8-9" or bigger. Your friend has SIX of them in what is essentially the same size tank!! It's going to be open warfare when they get bigger - if it isn't already.