Agreed that when one party is under 18 five year age gaps feel more ick but they quickly don't matter as much and how people meet and the context does matter, no matter the genders.
The biggest flags to me are if one person is under 16, if the age gap goes into double figures, or if the older person is in authority over the younger eg a teacher, police officer etc.
I do think it's a situation where friends and family should be watchful absolutely but it's a weird one as he's not exactly experiencing a regular 16 year olds life right now and they bonded through a love of darts online.
It's also a very new relationship, so she's unlikely to have known him prior to him being 16 (since he's almost 17) How many people under 25 give a shit about darts? I can see why they'd bond over that.
The volume of online abuse she's been getting is a hell of a lot for anyone to deal with and will affect them both.
Yes it's not ideal, but just let the kid persue his dreams and let his family worry about watching out for his well being.