TL DR: It might have caused the issues.
It could be what happened was a side effect of having diesel in the oil, if it was overfull (by what amount is not given) and/or if the engine was run for any appreciable time above idle. If it was put under load, it could have caused issues because diesel, while being an oil, isn't what I'd call good for an engine under load. Diesel will squish out of the bearing journals a lot easier than regular oil, which means it will have a lack of lubrication and may incur metal-on-metal contact.
I wouldn't think the diesel being in the oil (in and of itself) would have directly caused the issues you're talking about. The reason I state this is because diesel is actually oil, albeit a very thin oil and low in viscosity. It used to be very common to replace engine oil with diesel when changing the oil. You'd run it at idle for a while (a few minutes) to help clean out any gunk and the rest of the old oil. This would ensure you'd get a very clean crank case and the new oil would work as new oil, with mixing in very little of what was left behind. This is with a gasoline engine, though. A diesel engine has a lot higher compression, which means it needs more bearing cushioning than does a gasoline engine. All-in-all, not a good thing.
How any of this might have caused the head gasket to blow is anyone's guess, but it might have. There is really no correlation between the oil and sealing of the head gasket. The only way it might have caused an issue is through running the engine above idle, and then the engine overheating, which is a distinct possibility. If it wasn't run at all, then there would be no correlation, and no damage would have occurred.
