It's called runners feet and it is the main reason why I got into the nail business. Personally I do 115 mile cyclosportives -Wrynose or Bust- and I did suffer from delamination of the nail plate but not as bad as bruising. Improper shoe fit and socks are to blame. As long as it's not infected or the client does not suffer from onycholysis (nail plate lifting from the nail bed), it should be okay. In your client's case, it is the nail bed that has suffered trauma and the nail plates may lift. I'd leave it until it has healed properly and check that it hasn't lifted.
http://www.runnersworld.com/fun/ugly-runner-feet/slide/12