To be honest: I would view a dog like this as a rescue dog and this does not sound like an ethical breeder. If you want to take on such a dog it would be a kindness and I certainly would NOT pay a breeder for such a dog except to cover spay expenses (what is she doing still trying to have a 7 year old kennel dog have litters? Why didn't she spay her ages ago?).
If you are considering buying a dog, please have a read through how to find a breeder as the process whether for puppy or adult is the same: you want cardiologist clearances on parents and grandparents, ideally you want the dog to have been MRI scanned (for an adult) or its parents to have been (puppy or adult), you want the hip scores and eye certs as well (for this adult dog, not just the parents). If the breeder cannot produce any of this for a dog she has been actually trying to use in her breeding programme, RUN as fast as you can in the other direction.
Housetraining an adult is the dame process as a puppy but with caveats. It can be easier -- or it can be harder. Some kennel dogs will never be fully reliable inside and you must be willing to accept this. They also will not always be the best socialised dogs and it is much harder to address this as a remedial issue than to train a young dog. It is very rewarding to bring such dogs around to what should have been a normal life all along but this is not usually an easy task and you need to be able to make a full commitment to the dog, not view it as needing to meet some standards -- often this type of dog will never be a normal housedog and it is unfair to expect this.
If you are a first time dog owner, or feel out of your depth with a rescue dog (as this dog is going to be EXACTLY like the basic backyard breeder/puppy farm rescue dog if it is of unhealth-checked breeding and a poorly socialised, un-housetrained lifetime kennel dog
) I'd recommend looking for an adult cavalier that has lived inside with its breeder/family or a puppy from an excellent, health focused breeder who does have all the correct health clearances. This is a breed with some significant health problems, some of the risk of which can be hugely lowered by going to a breeder who follows the MVD protocol in particular. Plenty of cavaliers bred disregarding it die at young ages -- it is horrible to patronise breeders who do this to the breed and a heartbreak to lose your dog at only 5 or 6. Please take the time to either work with an ethical, health-focused breeder whose dogs are part of the INDOOR family, or else opt for a rescue dog. But do not pay someone like this for what is essentially a rescue case (and she KNOWS it). If you do decide to take the dog as a rescue case she should at the very least spay her before rehoming.
Make a decision with your head, not just your heart-- though I know that can be hard. There are many dogs that need homes; don't take on a dog you are not fully committed to for its lifetime regardless of difficulties and possible problems.