DNS/Routing advice

["Dan Lockwood" <dlockwood () shastalink k12 ca us>]

Everyone,
 
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!
 
Dan Lockwood