Sadly, I think your friend is right.
My first husband was my first ever boyfriend, and I was a great wife to him. I found out at 16 years in, that he'd been cheating on me the whole time. I stayed another 4 years and he carried on doing the same, even trying to sleep with my friends, so I left him. In that period that I was leaving, he beat me up twice, tried to swindle me out of money, and even tried to get sole custody of the kids!
My best friend in the world slept with him, so I lost her too.
My other two close friends didn't call me once to see how I was coping with divorce. Neither did my sister in laws, who I'd been close to for 20 years. Not one call, they just never spoke to me again.
Found out another close friend (my daughter's godmother) had also messed with him (not sure to what extent).
When my Mum died, I had to go through her e-mails to find out who her car was insured with (to cancel it), and I found several e-mails from my sister to my Mum, listing all the things she hated about me, and it contained several lies about me. I honestly thought we were best friends.
I had another friend assault me on a night out, because a guy she fancied seemed to like me.
I met someone after I divorced my husband, and thought he was the one. He got me pregnant, then said he didn't want the baby. To be fair, I didn't either, as I had 2 children already and I was getting older. On the day of the abortion, I got a text from him saying "I'm ready to get picked up Babe xx". He was 300 miles away. Yep, he was cheating too.
Another best friend had an alcohol problem, and accused me of having an affair with her DH. Purely because I was on his side in regards to her not driving her DD anywhere whilst drunk.
My Mum was lovely. But my Dad was awful throughout my childhood. A drunk bully who ruined almost every family event. I am now stuck doing loads for him, as he's immobile. He has softened alot, but it's a bugbear that I now have to care for him.
These experiences have made me very cautious of making friends.
However, I now have a lovely DH, who in all the 16 years we've been together has never let me down. My children are grown up, and are amazing people. I've carved out a little friendship group where I live, and they seem to be lovely girls, but my guard is still a little up. Me & DH have a lovely house at the seaside, away from the other places where previous things happened, so I don't have to worry about bumping into any of these wankers.
Sorry that was a long post, but essentially I agree with your friend. I watch alot of crime documentaries, and the things humans so to one another is truly terrifying.