Hi Wonder - I think we've all made that mistake on occasions! Adding yogurt might help, and the flavour is often toned down when it is eaten as part of a meal with rice/veges etc so the rendang is diluted with other flavours.
If it is really of the 'paint stripper' hotness I suppose you could (if you have time) cook up another batch of rendang without chilis and mix the hot one through - freeze extra for another day.