How Not to Multihome

[Keegan.Holley () sungard com]

I have a client that wants us to advertise an IP block assigned by another 
ISP.  I know that the best practice is to have them request an AS number 
from ARIN and peer with us, etc.  However, I cannot find any information 
that states as law.  Does anyone know of a document or RFC that states 
this?

Thanks,

Keegan