
Let the old girl be able to get away from the pup, so any interaction is entirely ion her terms.
Young pups like small children can make dogs unused to them very uncomfortable.
An extreme example, a friend had a Dobe bitch who didn't do pups at all,a and had been known to really go for one, but once they reached a more dog size she mothered and loved them.
Most just find pups irritating and can't see the point of them, as at that stage they are pests more than companions, especially with a dog who has been an only dog so not feeling the need for constant canine companionship.
My foundation bitch could take or leave canine company, on the other hand her daughter would not have been good as an only dog, never having been.