personally i would fill it to about 3/4 full with diesel so that it can slosh around and mix together when driving

it should be fine with just £10 worth a petrol in , if anything it will clean the fuel lines and injectors out

if he worried about it then run it until the 3/4 tank of mixed is gone then change the filter as all the crap in the lines will be trapped in the filter

he will probably find it starts and runs a bit better too ! , diesel additives are essential petroleum based anyway !