
Hi dogueone,
I would take the resident bitch to meet the new dog on *neutral ground* - say a park if possible (but not her regular walking area) where both couples can meet up and enjoy a restrained but pleasant walk with the two dogs and then go back to the house together :) I don't know if the dogs have met already (which IMO is always a better idea if taking on a rescue out of the puppy bracket) but if not then everyone has time to guage the reaction and/or interaction between them (if any at first).
It will probably take anything from a few days to a few of weeks before the rescue bitch's character comes out in her new surroundings so best prepare your friend to be vigilant for some time over meals, affections, treats, toys etc. Take gently it should hopefully work out fine.
Best wishes to them, Teri :)