It's not the fact that the tyres are wider that you have to worry about. It's the fact that the sidewalls of a 245/40R19 tyre are higher than those of a 235/40R19 tyre. Difference circumferences for the two tyres.
Not technically a good idea to have tyres of different circumferences on the same car, but in this case it would only be a 1,1% difference in circumference, which should be fine.
Also, if he fits only two new tyres at the same time, ALWAYS put the new tyres on the back. As in, take all four wheels off, put the rear wheels and their half-worn tyres on the front and put the former front wheels on the rear with the new tyres.
You ALWAYS want the tyres with the best grip on the rear axle, regardless of whether the car is RWD or FWD.
If the front wheels lose grip, what you get is understeer, which is easy to correct safely. If the rear wheels lose grip, you get oversteer, which is hard to correct safely and is particularly unexpected in a FWD car.