All of my UA flights landing into FRA this year have had the aircraft parked remotely and all of the passengers bussed back to Terminal 1. Once at Terminal 1, there's a set of monitors showing all connections and their gate numbers. If their connection isn't shown (within 4? hours) then ask the LH rep near the monitors where their gate will be (Either A, B, or C gates).
According to flightstats, it appears that the FRA-FCO flights are consistently leaving from the A gates.
Once up the escalator, they'll need to pass thru passport control.
What I can't remember (since I'm usually brain fuzzy after the flight) is after passport control, I believe you end up outside security and have to go back thru security to get to the A gates. I believe that is the case, but having a memory lag.