Considering the stuff DM pulled last time, I wouldn't be to happy about it either.

Not to mention that they also have some past experience with her as the GM...

"No, not DM..."
"So luke instead?"
"Ok, DM"

i can see sanji becoming gmpc somehow, and to avoid all powerfull gmpc, dm is the one dming on Sanji's fight

since they split the party from time to time, having a co-GM might be a good idea.