This story reminds me of what happened to me in my 20's. I had just broken up with my GF of a few months and later that day, she calls me and says her brother (whom I've never met) is coming over to kick my ***. I think to myself well this should be good, considering I have been taking martial arts since I was 4 and hold three black belts in 3 different styles and taught karate for a few years at my college, and at the time was working as a bodyguard for a national executive protection company.
I literally had blown it off after an hour to her being pissed, when all of a sudden there is this huge banging on my door, so I casually open the door and there standing in front of me is this 18 year old, who weighs a buck o five soaking wet and standing all of 5'6" tall. I'm 6'0 and at the time weighed 200lbs.
I look at him and go "yes" , He starts screaming, "I'm looking for Matthew" and I say very casually, "well you found him, can I help you with something" he gets this shocked terrified look on his face and stammers, "Um, yeah, okay, you ne-ed to stop seeing my sister" I then go, "um I broke up with her, that kinda was the plan there buddy" He then says, "well if you bother her, I'm going to have to do something to you! " I then casually say, "well, look I got nothing planned for the next few minutes, but if you want we can take this to the parking lot and I'll give you your best shot, but can we hurry this along, I gotta date coming over in a few. " He goes "F you and stay away from my sister. and storms off. he almost looked like he was going to wet himself.
Now I gotta give this guy props for protecting his sister, don't know why he got involved in the first place cause I broke it off with her, but oh well. I always wondered what would've happened had her brother been bigger than me, but oh well, to me people are nut jobs and this guy calling you sounds crazy, I mean calling a guy and threatening him with no real proof in this day and age is very stupid.
So take care and just be aware of your surroundings. Might help to take a self defense class.