You won't have any problems. Go ahead.
This comment is a shot in the dark ... 
Not knowing what hardware (motherboard/Power Supply) they have. Its best to ask people what they have for hardware and verify that the hardware will in fact work with no problems before just saying spend money you wont have any problems. 
Chances are if they are running a lower end GT 610 they probably have a newer computer which would support a faster memory video card, but if they have a really old computer and the lower end GT 610 with DDR3 is lucky to work to begin with such as a Pentium 4 or D computer with a PCIE 1.0 or 1.1 slot and then they buy a GTX 780 with DDR5 and all of a sudden it doesnt work or doesnt work out too well due to extreme bottleneck, Jatul28 probably wouldnt be appreciative that they sunk money into a system that the video card upgrade wasnt a good upgrade step for it, and a better system is then needed.
Some people come here with tight budgets of say $300. If they blew the $300 all in a video card but their system cant run it because its too old or power supply underrated
it would make your statement FALSE. Worst off is that they spend that $300 and then they have a computer that cant run it and now have to wait for money to become available to buy a heavier wattage power supply or even might need to replace the motherboard or computer all to begin with if its too old to run the video card properly.
And i want to know whether i can use ddr5 gpu on the same PCI-express slot?
This here is incomplete of vital information... what video card are they upgrading to. Most people dont upgrade from a GT 610 DDR3 to a GT 610 DDR5 if one even exists for an older GT 610 lower end card. Jatul28 could be looking at neeing a better power supply or computer to run whatever video card they are upgrading to with DDR5 memory. More information is needed before i would say yes.
