A regular teenage girl and a recently divorced 40 year old strike up a relationship only to realize that the 40 year old isn't who he says he is but he is really a sociopath

when an shy teen girl realises her recently divorced friend is a sociopath, she must become his femme fatale to stop him from hurting her family and friends.?

Why did it take 35 words to say "when a teen girl realises her recently divorced friend is a sociopath..."?

This logline doesn't tell enoght about what we will see in the movie: what happens when the girl discover he's a sociopath?

For me there isn't to much wrong with the logline. Maybe you can describe the protag="regular" girl a bit more.This could be a good psychothriller, but you have a weak title. I would just take
Dangerous Liaison(s) and cut of the "s". Do I want to root for a "regular girl"

A teenage femme fatale and a recently divorced 40 year old strike up a relationship only to realize that the 40 year old isn't who he says he is but a lovesick sociopath.

Now the protag is better described and she has a flaw, but for 'whom' will this flaw be fatal? I'm not sure who the protag is anymore?! When the protag? grows in character she gets less fatal and that weakens her position towards her brutal lover...

If someone is regular, you don't have to say it, it's implied. Same with typical.

Normally I say away from teenager as a description but since it seem relevant to the plot it is probably alright.

"When she discovers the older guy she met is a sociopath, a outgoing teen must find some leverage to stop him from hurting her family and friends."