NFPA 16: Standard for the Installation of Foam-Water Sprinkler and Foam-Water Spray Systems 2011 specifies the following rules for hydraulic calculations for foam systems:

7.4 Hydraulic Calculations
7.4.1 Foam-Water Deluge Systems.
7.4.1.3 Hydraulic calculations for piping carrying water or the foam solution shall be in accordance with NFPA 13, Standard for the Installation of Sprinkler Systems.
7.4.1.3.1 Piping carrying foam solution shall be sized as if carrying plain water.
7.4.1.4* The friction losses in piping carrying foam concentrate shall be calculated using the Darcy-Weisbach formula.

Performing a PIPENET calculation whilst following these rules is simple.  The procedure to follow is described below.

1. Foam Solution System:
This is a system in which the foam concentrate is already mixed with water.  According to the NFPA rules, this should be treated just like water.  In other words, the Hazen-Williams equation should be used for calculating the frictional pressure loss in pipes.  The density and viscosity of water are built into the Hazen–Williams equation and so the frictional pressure loss will not change.  If you changed the density of the fluid it will only affect the static head loss/gain.

2. Foam Concentrate System:
Typically a foam concentrate system is modelled on its own, and not as part of a firewater ringmain, because the density and viscosity are different from those of water.  According to the NFPA rules, the Darcy Weisbach equation must be used for calculating the frictional pressure loss.  So (a) you should input the pipes for the foam concentrate on their own and not as part of the firewater ringmain and (b) you should select the Darcy option and input the density and viscosity as shown below:

The Darcy equation uses the actual density and viscosity in calculating the frictional pressure loss.