Tbh, in my opinion this is a form of cheating.
Your friend should make his own account.
Kinda funny ....
. Worrying of being reported for sandbagging while you are confessing you were cheating.
Maybe you did not meant to do bad, but can you imagine if I would ask some GM to play some games on my account?
It would not be fair towards “my” opponents!

I've been playing these 1/0 tournaments in which my rating of around 1200 (bullet) plummets because I can't keep up with these insanely fast players. Once it hits below 1010 I'm (temporarily) banned from playing (I had the message of "sandbagging" a few times). Yet, I see other people playing at my level with ratings between 100 and 900 playing the same tournaments.
I’m with you in a similar manner.
I am experiencing 1200 as a tough spot in rapid and blitz because I do not feel established but competitive and experimental in the 1200-1500 bracket so I remain persistent.
Good job on bullet by the way. That’s an even tougher bracket to shoot for consistency. Seems like not all 1200 are equal. 1200 bullet > blitz > rapid from my 6 months of personal experience.