It looks as if the nail has had some trauma which has caused the bruise. It will eventually grow out, but it may also fall off. I looks as if blood is under the nail plate. You definitely should refer this person to a physician if it bothers her. He may be able to drain the blood by piercing the nail bed. If its not problematic then leave it go. It's not a very aesthetic looking nail, but you can just polish over it, or use some glitter to cover it.
Ouch! Agree with go to her doctor. You could polish over it but I'd try to avoid this for as long as possible because otherwise the client won't be able to see what's going on. From personal experience it is most likely to fall off. Get her to use some SolarOil to start the repair process x
It's going to take quite some time to grow out ... this is also the type of bruise that you will have to keep your eye on, it has quite a bit of healthy nail in front of the bruise and can have problems pushing the healthy nail.
If it's not painful, just let nature take its course.