A cat I know has had kittens about 10-12 weeks ago.
All of them but two have gone off to other owners. One of the kittens seems to have weened itself, but the other is still suckling. Should I encourage the cat to stop this? If so, how? The mother cat doesn't seem too bothered.

Does the mother stop producing milk or will she carry on if it is being taken?

Thanks

When you say one kitten is still suckling, do you mean that they won't eat any solid food at all? If so, this is a little unusual. Mine are just shy of 12 weeks, and all of them still have a sneaky suckle whenever mum's too sleepy to smack them for trying. It's very common that they do this until they head off to new owners, so I wouldn't worry about it much as long as the suckler is eating solids as well.