In cases like this, the vet probably recommended aborting the kittens because who knows what kind of shape or health the tom cat was in. He could have been diseased or a carrier of a disease. The kittens could end up being born with something, the mother could reject them or kill them and then you will be stuck bottle feeding them every 2-4 hours for a few weeks, if they make it that is.
Or everything could be prefectly fine. You just don't know. With the HUGE overpopulation of cats in the world and the ones being killed in shelters and everything, I would personally spay her asap which would abort the kittens. Cats are not people and aborting the kittens is nothing like aborting a child. The mother cat will not even know what is going on.
If you want to let her have them, you will need to talk to the vet about specific questions. Basically you need to:
-provide high/premium quality kitten food for the mother cat starting right now. examples being, Nutro Natural, Chicken Soup, Wellness...there are plenty more. And they need to come from a pet store not a grocery store or walmart.
-get the mother cat her shots when the vet says she can have them as well as all the kittens. I believe there are 3 sets of shots and certain weeks of age, but it could be two I do not remember.
-the mother needs to be spayed after the babies are born as well as ALL KITTENS before going to their new homes
-babies stay with the mother cat until 10-12 weeks of age. That is very important because even though the kittens may be more and more independent, they still have a lot to learn from the mother as far as socialization, litterbox manners, cleaning, etc.
-you will need to provide a nesting area for the mother cat. A quiet, warm, dimly lit small room. Away from kids and loud noises. Playing soft classical music around the time she is expectedto have the babies is a good idea. She will make the actual nest in her area of choice but she needs the space to do it and some towles and such.
-have some money set aside in case there is an emergency, the mother could need a c-section, there could be other complications.
-you will also need some toys and things and an area for the kittens, Kittens can be a mess. They get into EVERYTHING. Kitten proof your house. My spare bedroom was destroyed in the one week that I had kittens in my house.
-and be prepared, in the mean time, read up on cat pregnancies, what to expect, what to do, problems to look for. be ready to run out and pick up nursing bottles and kitten formula if something goes wrong. have a vets phone number on hand.
Hopefully everything will go smoothly. Good luck!