Interesting comments from everyone. I once had three much loved and established adult cats - 1 male and 2 females. A stray (approx 6 month +) turned up thin and battered at my villiage home and had struggled to keep going through the cold, wet winter..
After checking with neighbours that she was indeed a stray I got her to the vet who treated her and speyed her at my request. Cats are jealous creatures and to begin with they spat and growled at her. I made an extra fuss of them, almost ignoring the newcomer but each time THEY went out into the garden I gave HER lots of care and attention.
Mine were around 8 at the time and the four of them soon happily settled down.
FarmerD, it's good that you're thinking of your elderly cats needs. As Charlie is now an elderly lady maybe LizB's suggestion of a trial run with your friends kitten would be the best plan. If not, I'd go with leaving things as they are.
All the best...Lovelight x