I had one of these a few years ago, SGB - he befriended me, we became good mates. He came across as a bit needy & inept, but seemed like a nice chap who just struggled a bit to make friends on his own...
He got it together with a friend of mine, which I thought was great for them both (both shy, had previous crap relationships).
THEN he started obsessively pursuing another (married) friend - someone both his gf & I knew well. She initially rather encouraged it - wobbly marriage, was flattered - then realised she was being an idiot, told him to get lost & ultimately, when he persisted, told his gf what he was up to.
The gf dumped him, & months of keyboard warrior hell broke loose - he found a wide assortment of ways to annoy & distress his ex-gf, the woman he'd been chasing, & me for introducing him to them both in the first place.
Lots of setting up of webpages to slag all three of us off - photographs of nights out with inappropriate/offensive captions as to what he claimed to have got up to with one or all three of us later - threats to contact partners or employers with similar tales - veiled 'watch your back walking home these dark nights' type threats...
I know where you're coming from about it seeming 'wussy' to go to the police (with our pet loon, the other two women eventually did feel they had to & I supported that & made a witness statement).
However, there's a line being crossed here - arguing with someone online is not the same as threatening to bray them one in RL.
It's escalating, too, if he's threatening your friend. I think you now need to talk to the police even if only to support her - this was the situation I ended up in, that although I wasn't honestly intimidated by the loon, my friends genuinely were.