Dear Simone,

first of all place your bonsai in a well-ventilated place and stop watering completely; if possible try to remove the mold and the soil in which it sinks. If it were possible it would be better to repot even the bonsai. You remove it from the jar and wash the container thoroughly, letting it dry in the sun; the roots will be dusted with a generic fungicide; then the bonsai will be repotted with fresh, good quality soil.After having done all this perhaps it is appropriate to thank the friend with a beer, taking advantage of the evening to explain to him how much you can water a ficus bonsai in winter, so that this incident does not happen again.