The fact that it was working before your friend got a hold of it. The caps had to be installed correctly.Check it like 4 individual diodes 1.AC1---- pos , 2. pos ---- AC2 , 3. AC1---- neg, 4. neg ---- AC2..
of course measure the forward and reverse of each diode.
Due to being connected directly to the power transformer you need to check it out of circuit.