well like @deadly mention servicing the radiator might solve ur frd
s prb, but there is one more thing, do ur frd have to fill up the radiator each n every time b4 riding the bike ?? if it is then the possible cause will b the damage of seating area of the radiator cap, check that out may b its not sealing and the water is leak off from the expasion bottle. OR check the RPM of the cooling fan, cuzz sumtimes the fan is running but not on sufficient RPM to cool the water in the radiator. and abt the head gasket !! ask ur frd to cold start the bike and remove the radiator cap and give race, if ur frd sees water bubbles coming out from the radiator then gasket may b faulty elsewise u dun need to go for the gasket
rest @foosa can help u quite better.