D.
Personally, if I am the administrator / owner of a forum then the buck stops with me. It sounds like this member has been around for a long time and is turning to you for help, not just winging. How long has the other member involved been a part of the forums and what kind of contributions have they made.
I think that if a member goes out of their way to help someone and they are disrespected for that then the helper deserves the backing and support of the admin / moderators, not to be brushed off with a whimpy and patronising, "oh well never mind, ignore it and it will go away" attitude.
I would be very diplomatic about it and approach the person responsible with the view that you have read their comments and think that other members of your community might be offended, upset, insulted, etc. by them and to bear in mind that this is a public forum where everyone is accepted and no amount of bullying, intimidation, disparagement, etc. will be tolerated.
You can't just sit back and watch, it will spiral out of control. Act now, show them who's boss, but be fair not ruthless, and prevent any further escalations. Do this and your members will respect you. Leave it to play itself out and you'll be a loser in more ways than one.
