santana - It would be helpful if we knew more about your friend's RV. Is it a motorhome, and if-so what brand (Winnebago, Fleetwood, etc.)? Also, is it a Class A or C and what chassis is it on (Ford, Chevy, Diesel)?
For the purpose of a quick suggestion, I'm going to "assume" it's a Class A or C motorhome on a Ford or Chevy chassis. Most battery shut-off switches trigger a solenoid to disconnect the battery. Because power is needed to actuate the solenoid, it can be difficult to turn the battery back on if the battery is dead. The first thing I'd check is the engine battery voltage. If it's below 12v, have it recharged and try again.
Most motorhomes have two or more batteries, one for the engine and one for the coach (house). His RV probably has an emergency start, or auxiliary battery switch on the dash. This switch is used to enable you to start the engine with the coach battery if the engine battery is dead. Once started, the engine alternator can then simultaneously recharge both batteries. So, you might try this.....push AND HOLD this switch while attempting to start the engine. If this works, continue to hold the switch for 30 seconds, which should be long enough for this solenoid to trigger and start the recharging process. With the engine running, you should be able to reset the battery disconnect switch to ON. Good luck. - Dave