I am sorry to say your friend's car has fallen victim to the dreaded Nikasil engine cylinder liner failure. This is in fact a well known flaw in the 93-95 4.0L V-8 engines from BMW. BMW offered replacements under an extended warranty on these cars for many people but unfortunately they chose not to notify the owners of this flaw, so the only ones that were covered were early failures or people who found out from other sources.
BMW -NA may still help this person out if they bought the car new especially. Tell them to go to the selling dealer and complain about not being told of the flaw and ask them to cover it under the extended BMW warranty for the engines. They will either do the job for free, offer to split the cost to some degree, or refuse. If the later, then they are no worse off than theya re now, but my advice is to go through a BMW dealer.
Complain to them about rough idle and request they do a leakdown test on the engine. If it fails BMW's sepcs, they may come up with an offer.