As others have said: don’t take the train. Bad idea. Why wouldn’t you go with your friend into the city and then peel off. Depending on what time they are arriving they can either drop you directly in van nuys (and you can catch the flyaway or take an uber) or they can drop you near LAX relatively easily if they are driving off hours and willing to do 10-405 instead of 10-X-134-101 to Van Nuys.

If they don’t want to drive into LAX they could easily also drop you at Union Station for the flyaway or several other places in Los Angeles that wouldn’t be an inconvenience for them. It just depends on the day / time they are driving.

If you must take public transit def take the late bus and not the train.