Any Sim that is eaten by a cow plant again while they still have the 'Drained' moodlet will be completely consumed by the plant 100% of the time and it will provide the life extending milk. My current Sim has personally fed on the Life Essence of no less than 13 Sims (tastes like Soylant Green.).
I found a page when I looking for help for finding a tomato plant and onion plant. It tells you what plants you need to graft together to make other plants it also has a lot about the cowplant on there. Unfortunately I am a new member so cannot post a link. But its called carls sims 4 guide hopefully a member will be able to find it an then post the link. I realize this thread is pretty old, but since it's had a second coming and the page that Wenni mentions really is very helpful, here's the link.
It is snapdragon and dragonfruit for cowberries. However, you need to graft snapdragon and strawberry to get the dragonfruit. Check the guide at Carl's Sims 4 Guide for help in locating plants. The snapdragons and strawberries aren't actually growing at the BFF house, they are in the common area garden behind that house. You don't need to water them, there's an NPC gardener who should be doing that. You can also find snapdragons growing wild in that neighborhood. If you can't find your snapdragons or strawberries in the BFF neighborhood then look in other places.
They can be found in several neighborhoods, growing wild or in community gardens. You can find both plants in the community garden behind Umbrage Manor, for instance. Keep in mind that plants only grow while you are in their neighborhood, so you will usually need to spend a day or two in that neighborhood before the plants are ready for harvest. Fortunately, you don't have to spend all that time in one visit. You can make several visits of a few hours each, as long as enough total time passes in the neighborhood. ETA: A much easier way to get cowplants is by fishing near the cave at Desert Bloom Park in Oasis Springs. It's pretty easy to catch cowberries there.