Question: What is a karmic debt? Will it surely ripen in this lifetime?
Answer: A karmic debt is simply negative karma created that has yet to ripen in effect. Its effects can be mitigated with positive karma, as can be seen at http://thedailyenlightenment.
Question: An old friend nearly had a fatal accident. Was it due to a karmic dept about to ripen?
Answer: Having near misses do not surely mean one is near death or disaster. If there was no actual suffering or much of it, this means there was no serious negative karma ripening then. However, what could have been serious negative karma’s ripening could have been averted. Yet, this does not mean there will not be any more problems later. Even for one who did not have a near miss, an accident might still karmically manifest under the ‘right’ conditions suddenly. It could simply be, however, in this case, due to faltering mindfulness of the environment, that led to the near miss.
Question: How can I help him, who is a non-Buddhist?
Answer: Step one is to ask him to always be more mindful of his surroundings. If there is a situation he is stuck in, he should shout for help quickly too. As you cannot dedicate mindfulness to another person, this is something that he has to do personally. Step two is to encourage him to do more good to creates positive and thus protective karma. You do not have to use Buddhist terms for encouraging this. Step three is that you can do more good and Nianfo to dedicate merits to share with him, for mitigation of potential accidents, if any.