Tagalong for mission complete unlockable weapons

I have the "Mole point bravo" arc from Tech Naylor and I want to get my alt some rularuu weapons. Does she have to have the mission herself or can she just tag along with me?

yep she can just tag along.

To clarify:

If someone is hunting eyeballs for the Visionary badge to unlock the weapons that way, then you can tag along. This works because defeat credit goes to everyone on the team - so if you are on the team and you are on the map, you are getting credit for eyeball defeats whether you personally defeat them or not.

However, if you're trying to unlock the weapons by completing an arc or TF, then you have to actually complete that mission on that character. In this case, every character who wanted to unlock the weapons would have to have Naylor's arc and co-complete if they also wanted credit. So lowbies would not be able to tag along and get their weapons unlocked this way as they wouldn't be able to get Naylor's missions.

Does Flashback mode count an arc as belonging to everyone on the team as a TF does?

I'm pretty sure the Ouro TFs do, and that they're just using normal Flashback setup, but I've never paid attention to the few times I've used regular Flashback while teaming (I almost always solo those).