Le/Lui question from Duolingo

Sentence I was given: Elle s'est souvenue de lui quand elle l'a vu.

Why not: Elle se l'est souvenue quand elle l'a vu.

I'm not sure what the difference is between using "lui" and "le" here.

Also, why not: Elle lui s'est souvenue quand elle l'a vu. Does "lui" have to come after the verb as in the example?

Thank you.