Oil leak

I have zero knowledge on cars but here goes; my car is always losing oil, suspecting a leak somewhere, but on one occasion my partner was driving my car and the oil light came on. he topped up oil etc and the car started smoking and there were flames coming from under the bonnet. The oil cap was off so do you think he left the oil cap off which resulted in the smoke and flames or do you think it is something else and that has caused the oil cap to pop off? He thinks he is pretty sure that he put the oil cap back on.