DNS/Routing advice

I have a customer that is multihomed, to a public ISP and to another large network that uses 10.0.0.0 address space. The private address space also has services available via public address space and consequently is running a split DNS service, public and private. Because of firewalls and the placement of DNS servers this customer has a nasty routing situation and in order to make DNS work for the private numbers, has spoofed the domain of the private network. My question is this: are there any documents or RFCs that outline what is an acceptable practice for running DNS and what is not? Their kluge of a network causes continuous problems for both the upstream ISP and the private network to which they are connecting and we may find ourselves in a situation where we have to say that 'xyz' is an acceptable way of operating and 'abc' is not. Any advice is appreciated. Thanks!