IMO, no problem. The EWR flight is a 343 and there aren't many ex-EU * arrivals at that time, so there shouldn't be too many people. Your friends will have to go through immigration and screening, as said before, but in my experience staff there is efficient and there are enough lanes. Total walking distance gate to gate should be no more than a couple of hundred feet.

Due to swine flu your friends will have to fill out forms about your health and turn them in to doctors waiting at the gate on arrival. They should have those filled out and ready so they can just hand them in and proceed.

DUS is a pretty decent airport to connect as it is a lot more compact than the big hubs.