thinner should ideally be added to ensure the viscosity is correct, different colours and pigments often need a different thinner amount to get the viscosity time correct, thats why mixing rooms should always be at the same temperature as this massivly effects viscosity. i cant bloody remember the standard mixing room temp, 80 degs i think

if he kept the tins sealed, im sure it would be ok for a week or so, a better option than incorrectly mixing the paint each time lol

pmsl at the kitchen scales