INdia is already planning for chinese 5th gen jets so pakistan getting one wont make any major difference to indias future plans but if pakistan got one in this decade then india wilĺ need to fast track the fth gen plans .
Most notably india is going for home made 5th gen jet but that is atleast 12 yrs away from now minimum so most likely india will be behind pakistan if j31 arrives before 2035.
ONly viable solution is buying from a foreign vendor either russia, france , usa or maybe south korea. Russian su57 can be bought if chinese 5th gen arrives as both can have similar technological level.
f35 is not possible as india does not trust Usa for such an important purchase that too against americas old ally pakistan.
Most likely scenario will be more upgraded rafale from france.
And in very rare case india can also move to south korea for assembly of kf31 if the cost is reasonable.
PAkistan getting 5th gen wont give a decisive edge to pakistan but definetely put pressure on iaf to act faster.
And it is given at some point chinese will definetely deliver most of the chinese second tier equipments to pakistan to contain india. And pakistan will accept anything for free.