A slight follow on from the car I am meant to be picking up

Seller started it up yesterday all fine. Tonight he tells me it is spinning but not catching then slowly dieing. Has tried another battery and its the same

What should he check? He has put carb cleaner in it, but surely that would stop it running if too much? Always lowered the revs when I sprayed it in other vehicles.

Any quick help would be great

Ok. he has now got it started but I am worried it could go on me on the way home. Fuel relay? Pump? Obviously it has spark now. Ignition switch even?