I can't call her SBW or TAP, he's probably get defensive. And OW or mistress probably would just make him feel guilty, which would come out as angry/defensive. So....."Her" it is lol.
LOL! On the rare occassion I've had to refer to the aubergine, I've tried to squeeze out a 'girlfriend' but only managed to get as far as 'your gir', and then had to switch to her ~#'~/~ name 'your gir Sally' which sounds somewhat like 'your grrrrrr silly' through clenched teeth! 'Her' is a good alternative. I'm going to try it!
