Perhaps the alien's script in the dream is being interpreted incorrectly.
"Well, that surprising, here you see a vehicle skidding down the road side-ways but you are shocked to see me. Just wait till you see what happens next! You aren't going to believe what happens next!" it relays as it keeps walking up the embankment.
Perhaps next refers to next, as in after the event. After the unfortunate death of the driver. Perhaps your mother won't believe what happens next, after death. The alien seemed very excited about it.
walking up the embankment
Perhaps the alien has a duty to do after the driver has passed. "Walking up the embankment" seems metaphorical.
Perhaps your mother was really never meant to see the face of the driver.
Perhaps there is a plan to life. Sure does feel like it sometimes.
And while not allowed to interfere with it in a waked state, we are allowed some form of coping by receiving signs like these, in the hopes that we can recognize that there is a predetermined course. She did, after all- see the event before it occurred.
Again, my condolences to you and your mother. Time heals all wounds.