Obviously some people think this is a far fetched idea -- and to some degree it is. But newspapers carry at least one or two accounts every year of the cops bagging someone for wanting to have a spouse whacked by some 3rd party. We know from experience this means at least 3-10 times that many actually occur - still it's a pretty small number, methinks.
It's much more common for the ex-spouse to try to do the job themselves. And it's not outside the realm of possibility that your new girlfriend's former significant other has decided to take out both of you.
I spent the time writing the previous post because I've "BTDT" without getting the T-shirt, for a friend.
Not too many years ago, I found myself in a similar situation. A lady friend's ex decided that the two of us must be an "item" and decided he was going to take us both out. His downfall was letting her know about his decision ahead of time, via phone, and she managed to record it. Cops were sympathetic and unhelpful when it came to defense other than to keep the doors locked and "Call 911 if he shows up". Thanks a bunch.
Managed to get her a R.O. which the judge, in chambers, told her lawyer that it would act as her CCW. Helped her buy a very nice used Model 65 and got her to the range four times for practice. He showed up when she wasn't home and left her a nice "reminder" -- a knife through her photo on her pillow. The next day her ex tried to get her daughter out of school and failed. Third day her car sprouted 2 flat tires at work through the sidewalls. Daughter was sent to her Uncle's house, the PD notified in each case and they claimed they couldn't find him. She rented a car and stayed in a hotel near work for 4 days. I picked up her mail from a helpful neighbor, made sure I wasn't followed and gave it to the hotel in a manila envelope for her. At the end of 4 days I had managed to spot him leaving his work each day and gave that info to the PD. On day 5 they stopped him and since he's 6'3" 210 lbs they used four cars. He fought and lost. During the time between his arrest and trial, he tried again, this time with a molotov cocktail on the front porch. Neighbor identified the guy & car to the cops and he'll be out of jail next year.
As a couple of side notes...
Never tell your boss or your landlord of these things in advance. She told her boss and after the tire puncturing, which she reported to her boss, they told her that any further incidents of "violence" and they would have to let her go "to protect their other employees". Nice, huh?
She did tell her landlord too, so he would be aware since he was refurbishing his other rental across the street. After the porch fire (a new front door, moulding and some paint fixed it) he raised her rent almost 60%, effective in 3 months. His excuse what that his insurance rates went "way up" because of it. She managed to negotiate a delay until summer so she could change school districts for her daughter then moved closer to her new job.
You'll notice that I've left out what I did as defensive measures. No need to go into that other than to say that staying alert is key, along with using your head to think through the situation.