Many people have this issue. There is no problem. You may want to get a letter from your doctor signed and dated that your Mother had a metal plate installed recently. Either way, she will be stopped and hand-searched. They will pat her down, take off her shoes and use the wand. If they dont find anything other then the metal plate (with the wand) then they will let you both on your way. Just make sure she doesn't have anything else metal on her because that may trigger a full search.
Many people have this issue.