Well, there are battery powered Cpap machines that might work for you.
What you need probably is an INverter. That takes 12VDC and converts it to 120VAC. Depending on the amp draw of his machine a 1000 watt unit will probably work just fine. INverters need to be placed as close to the battery bask as possible. Then its a matter of adding an inline fuse between the batteries and INverter and using sufficiently large gage wire to carry the load. From the INverter you can simply run a dedicated line to new receptacle near the bed.