Extract the meat from the sauce, and wipe the meat to remove traces.
Cook lots of waxy potatoes (waxy so that they don't break up and spoil the sauce) in the sauce.
Throw away the potatoes and test the sauce.
The technique is that used to remove salt content, but it /may/ work for chilli content.
Alternative : add as much natural yoghurt as you possibly can to the sauce;
yoghurt is a natural ant-chilli agent, and will definitely take the edge off the burn.
Maybe even both, in that order.
** Phil.
P.S. When is the birthday party ? All curries moderate their own heat with prolonged storage, but not necessarily to the extent that you need.