As far as I know, the carryon restriction is implemented by boarding group. That is, if you board with group 5, then you don't get a carryon -- if you board earlier, you get to take a carryon. Generally, if two people are flying together and one has a higher boarding group, they are informally (but reliably) allowed to board together. So if your *G status puts you in Boarding Group 2 (or whatever) despite the BE ticket, then your companion will be able to walk up to the gate with you during BG2 and board with you -- and take a carryon bag.

I could be wrong about several parts of this -- I have no personal experience with it! I'm not 100% certain that *G status gets you a higher boarding group on a BE ticket, nor am I 100% certain that a BG5 passenger boarding with a companion in BG2 will be allowed to take a carryon.

And the checked bag question is totally separate, because that's a formal process at checkin. So United's official policy (whatever it is) is much more likely to be enforced there.