What is the function of nitrobenzene as a solvent in Friedel Crafts alkylation reaction? I would like to know what is the function of nitrobenzene as a solvent in Friedel Crafts alkylation reaction. 

Answer to this question suggests that Friedel Crafts reaction is not possible with nitrobenzene and I understand that it's because of the big loss of electron density of the benzene ring due to the highly deactivating nature of $\ce{-NO2}$
Without any more information, it's just that, a solvent. Since it is a poor substrate for Friedel Crafts alkylations, it can be considered inert in these conditions. The choice of nitrobenzene would probably be to carry out the reaction at a relatively high temperature, since it boils at 210.9 °C.