The former owner created (or had someone create) an unsafe trigger in that rifle. Ruger triggers are not adjustable - so someone has re-worked the standard trigger to lighten it - and gone TOO far. ANY bolt-action rifle that will slam-fire (go off as the bolt is closed) is VERY dangerous !!! DO NOT shoot a rifle that will do this !!! Whoever did the "trigger job" didn't properly test for safety - and thus created an accident, waiting to happen (as has ALREADY happened). PLEASE DEMAND that your friend have this rifle repaired, BEFORE ever shooting it again.
The real risk is that, next time, the round will actually go off, BEFORE the bolt is fully locked (in "battery") - and this will most likely destroy the rifle - and may well severely injure (or kill) your friend. This is NOT something to take lightly !!!

If the original trigger has been so- doctored - it is ruined. Fortunately, there are aftermarket replacement triggers available. A Timney adjustable trigger (or several other brands) can be installed, which is adjustable and can be made quite comfortable in use, while still being safe. These are not terribly expensive.