well, as you know the official deck max is 60 and the deck min is 40. from my personal experience, as well as the opinions of some of the top players in the game, I have concluded that the 4 magic numbers, when it comes to deck size, that everyone should stick to is are 40, 41, 42, and 45. there are, however, a few decks that can skew from these numbers and still be viable (depending on the strategies of the deck). in the case of your friend's DD deck, 58 probably is too many cards. I run a Macro Monarch deck myself, and the main deck is 40. for your friend's deck, I would suggest he stick to around 40-42 cards, but without seeing a deck list and an explanation of the main strats of the deck, I really cannot make any other suggestions beyond that. I hope that my explanation helps.

id say post a decklist (if you want) to see what is viable and what should be pulled out.
for me its either 41 or i dont run it idk why x]