First stop before you start considering expensive items like turbos and injectors is an EGR valve check - a code read will reveal any problems, but it is easy enough to do a DIY inspection and clean/replacement, check the pipe serving it for splits too - there's plenty of info on the internet.

There's also a known problem with the injector working loom chafing on the top pulley, so check this too. This also manifests itself with a general injector fault code.

Get him to take out the EGR valve (very easy job) and clean it the other common fault is the turbo actuator or get him to put Aa couple bottles of commercial Fortes cleaner in the tank it sometimes resolves the issues for a few thousand miles.