1) your problem
I'm not sure if I can ask for protection for someone else, or that they need to join here to request it themselves, I'd be happy to get them to do that but I'm sort of exploring my options right now, any advice would be helpful.

The problem basically started after the group that was working at the Club/imvu room splintered into several new clubs due to drama within management. One of the ringleaders of the group is harassing my friend by repeatedly going into her and other rooms and giving out her personal information like phone number and rl-name, he obtained these through one of the girls he intimidated in coming along with him. By now she's moved on elsewhere but he continues to come in and talk trash. He has already left her several threatening voicemail messages up to where she had to change her phone number to get rid of it, now he keeps going on about he'll track her and her new phone number and and in general just being an ass. She has already reported him repeatedly to imvu but of course they cannot do much.

2) how this problem effects you
It doesn't affect me directly, thankfully. I had him blocked quite a long time ago already and do not give out personal information like that, but it does affect our group as such as he's intimidating the room and makes people feel unsafe.

3) Where in IMVU this problem is coming from
I will not put down names here, but happy to provide them if need be. In short it is coming from two accounts owned by him and one account from his lady-friend who has no qualms sauntering into the room after he left to sprinkle some salt in the wounds.

I do understand the root cause of this is that she gave out this personal information in the first place, but everyone went their own way after the group broke and he has no right to bully her like that, I'd be pretty scared too if it happened to me.

Your friend can talk to her phone provider about changing her phone number. I've done it before myself ... a clean sweep on your phone is healthy for your social life, scrubs away all the unwanted filth.

I'm assuming all of these things have been done but just in case they have not, have your friend:

1) Block both of his accounts and his lady-friend's. If he creates new ones, block them too. They can still enter rooms she is in that she doesn't own so she should probably stay in rooms she owns or be made a mod in those she doesn't. They can't enter those if they're blocked or she can boot.
2) Keep chat logs and screenshots and submit them when she flags his/her behavior to IMVU. I disagree that they can't do much. They can and will ban him/her for a week or a month or forever for repeated violations that are substantiated with logs/screenies.
3) Have all of her friends corroborate her story to IMVU by also doing step 2 above.
4) I see you say she has already changed her number. Him talking trash about tracking her new number is just that, trash. She keeps him away on IMVU by blocking, he can't talk trash to her. Problem solved.
5) Any other social media she may have contact with him on FB, Kik, Skype, Discord, etc. do steps one & two on those sites as well.

PITA I know but those are some options for the protection part.

Now if it's revenge she's looking for ... get his IP address and submit it -anonymously- to local law enforcement as a source of kiddie porn. If he doesn't have any, he'll be fine but thinking about them tearing his PC apart looking for it ... hehehe ... priceless.

Screenshots are not accepted by IMVU staff, unfortunately.
You'd have to be in the room and endure the abuse to flag it. A real flaw.
The more people you can get to flag this person, the better though. As previously stated by Soulljah.

I am not sure if IMVU actually blocks IP addresses or not (I don't think so)
Alas, all you can do is just endure his little tantrum, do as Soulljah guided and eventually his steam will cool off.

If not, have your friend make a new account to vacay in until it feels safe again, I will happily provide her with a few to buy outfits and such