I think at 14 it's time he sleeps alone.
I agree with bhenneker I dont see any harm if its on visits , its probably for company, when hes not at home ..
Does he sleep with her every night?? or is it just once and awhile??
my son is 7 and sleeps with my parents when ever he sleeps over there(which is only maybe once every two months). I don't see a problem with it as long as your son is okay with it. but if it is an every day thing i would put a stop to it ASAP.