Wait, are you saying they would use it to prep and cheat in adjournments, or are you saying they would use it to play for them like B.Ivanov.
If they used it like B.Ivanov, they'd say the player is a ridiculously good calculator... and when they can't reproduce any of it in analysis it would be really fishy.
Let's say one of today's strongest chess engines, Komodo or Stockfish went back in time (let's say to the 60s or the 70's) along with all the opening and endgame databases. Let's say that this was available to just one person, so that he can cheat without anyone ever finding out. Soon enough, he'll become the World Champion.
What would the other grandmasters say about his playing style? Would they claim that it's wrong but works for some reason or would they glorify him?
EDIT: He also would have the ability to reproduce variations during post game analysis. If someone asks about a move, he would input that move into the engine (somehow) and spit out the variation.