Is it advisable to withdraw bulk amount from GPF account of my father, due to retire in 5 yrs, and invest in MF in order to get monthly income once he retires?

Without knowing the overall portfolio or investor profile or post retirement income needs, a thumb rule for you is to continue PF investments as long as possible. However if you don’t have any equity exposure it makes sense to add it bas ay 5% to 10% only (out of GPF kitty if nothing else is available).

Once he is ready to withdraw GPF, then invest this in amit of debt and balance mutual funds to meet the income requirements. (add some equity very carefully)