how to respond to a demand letter for child support?

Hi a close friend of mine, just got a demand letter from his previous partner. The amount that she is demanding for the support is unreasonable to the father's end. His reasons were, before he is giving a certain amount but he said that the money that he is giving is not fully consumed by his son. Like, napupunta sa pamilya nung babae. Just want to know how to respond on a demand letter, because she also demands an arrears from the time that my friend, stop giving him money. Also, the reason why he also stopped giving support according to him is that, pinahiya siya sa facebook before when the child got sick and he just gave Php.500.00, only because that's the only money left on his wallet that day. Thank you. Looking forward for an immediate response.

Hi, Can someone give an advice? Thank you.