I agree with Wally (and your mom). Stay away from this horse because, as Wally said, navicular is a degenerative desease that will only get worse. This horse will require a lot of time and effort, not to mention money. I assume since this is your first horse you are wanting one to run and have fun with, and a horse with navicular is not going to cut it. I would be especially wary since the owner is so willing to give him to you for free. If she really wants the horse to go to a good home and doesn't care about the money, why not give him to someone as a companion horse where he will not be ridden. This horse may be ridable for many years, or you may get him and a month later he goes lame and can never be ridden again. How old is this horse? If you really have your heart set on this horse (I hope for your sake you don't), maybe you could just lease him with the stipulation that you will keep him only as long as he is ridable, and then when the navicular gets worse you can return him. Regardless of what you think now, if you get this horse, you will become very attached to him and it will be absolutely awful when the day comes that he is no longer ridable. For your own sake, I would suggest you wait and save your money until you find a horse that truly suits your needs.
Allie