All of the static IP's that are assigned to the account would be available from the modem/gateway. If the modem/gateway only has 1 RJ45 output, like most, then you can easily drop a small network switch in between the modem and yours/his routers. Then you would assign one of the statics as your WAN IP and another for his.

This way when his network shuts down, yours is still running smoothly and completely separate from his.

What kind of modem did they provide? the CG3000DCR? If that's the case, then you should be able to just plug in to one of the gigabit ports and assign your 750 one of the static IP's. Provided that Comcast has actually set up the block for that modem.

Also, when you say he shuts down the network at night, does this include the modem? Cause it shouldn't.