Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

It might be nice if the BRB or Eldar codex clarified, but it's not necessary.

You have the standard unit cohesion rules.
You have the exception for vehicles.
You have the exceptions for flyers and skimmers.

You have Jetbikes, which are not vehicles, let alone flyers or skimmers.

Follow the rules you have.

If your Eldar playing friend wants to claim the Jetbikes are allowed a special exemption, have them look for it and show it to you.

I don't play Eldar. The Jetbikes may very well have some sort of super secret space-elf cheese that allows this.

I do however see a problem in your view of this situation. When a player claims their units can do something that breaks the BRB general rules and you question it, it's up to them to show you why they can. It shouldn't be you showing why they can't

Reminds me of the guy at my FLGS that kept trying to tank shock with his SM bikes 'cause it made sense that he could.'