Not in a cow

. But a couple of times in people.
I surprised he didn't drain it. It is an old addage that the treatment for an abscess is drainage - cut it open and drain it. Thats what I thought had been done to the quarter that was out when you got her - although I'd never cut it all the way down to the nipple in a human.
Anyhow, I guess your vet is expecting it will drain on it's own, which generally they do, but meanwhile she's sick, and can get sicker. I've never watched and abscess in a human. It's faster to drain it - decreases healing time and pain, and you can control the exit point - which in this case you would want to be as far down (so it drains well) as possible.
The key with draining an abscess is that you need to have a large enough opening that it wound just close up at the skin before the abscess cavity has time to heal. In humans, in areas where you need to avoid a larger scar, I will use a smaller incision shaped like a + with the 4 tips of skin cut off, and place a drain. In an area that don't show or with larger abscesses, I'd use a larger incision shaped like an elipse. Generally I pack these with gauze and change the gauze twice a day, but I don't expect a cow would let you do this, so I'd just let it drain.
Sorry for your troubles.
How is that little heiffer doing? Can you post some photos?