Yes, you could buy it some milk. (Select the ability to feed the milk to your friend's cat from the milk's menu, by touching the bowl of milk. Otherwise it will not allow your friend's cat to drink from it.) Other than that, your friend must hold the cat for half an hour or more.
as maxwell said, kittys gain happiness from being petted or held.
and cuddling your kittys for half hour a day (i call it kitty cuddle time!) does the same thing as milk. but petting anyones cat but your own will not bring those cats happiness up so they need to be held by they're owner.
Increasing Happiness
