It sounds like the accident would not have happened had Person B not been driving on the wrong side of the road.
OTOH, had Person A not been speeding,
maybe Person B would not have
died.US Whenever someone is involved in a death like this, they feel terrible. For example, co-workers, friends, and relatives of someone who commits suicide all generally generally express regret for "not having some something. This does not mean that they bear some guilt or fault—they might, if the person confided in them and they just blew him off, or if they had bullied the person—but overall, no.
Sometimes bad things happen. We want to feel a sense of control over our lives so we try to think of what we could have done for the bad thing not to have happened, but it is just to regain a sense of control over things we don’t control.
If Person B is or becomes Catholic, he can go to confession, and know for certain he is forgiven. But but what will be more important is for him to reliquish the event to God, to, in effect, forgive himself.