I want to play with my friend and I have both the upgrade pack DLCs. I'm level 65 and my friend is level 61. If he doesn't have the Ultimate Vault Hunter Upgrade Pack Two, will everything still scale to me in his game? We're playing on UVHM.
Playthrough One and Two (normal and True Vault Hunter mode) are scaled to the host's level or the host's story progression (based on whether or not the final boss was killed). Ultimate Vault Hunter Mode scales to the highest level player, regardless of whether or not it's possible for the other players in the match to go that high.
If you intend to make a lower level character to play with your friend, I strongly recommend using Gibbed's save editor rather than manually boosting a new character to save yourself a few hours of pointless grinding.