Your best friend in deep trouble. If 1 day the girl knows where he & family lives & pays a visit.. habis lar...

Since you are sincere enough to help your good friend here, here some suggestions below;
1) First thing first, your friend need to stop all the flings with her. If still continue & the girl don't take pills, once pregnant, its deep shit condition. Uninstall the tinder & change no, directly no contact with the girl at all.
2) As shared by other /k, she might addicted to s*x rather than your friend here. Your friend has heighten her s*x levels & pleasures & she looonnnggg for moree. That why she so obssess with him... 1 way to deal is, a subsititude(black goat) take over & divert her attention.. As few /k said, intro the info (PM) to them. Maybe some /k can provide much better treatment to her...

Thats what can think off.. if he don't do the above, surely & definitely he will destroy not only his family, but also the girl & ppl around....

The girl got mixed up between s*x & love. For men, s*x is s*x & love is love. She though since she really love his bed performance means she really love him....
Now all already quiet down & he don't seek your advice anymore, means already ok..

No news is good news...