Is flashback a good opportunity to make him familiar with mechanics since there will be a mix of almost all previous leagues or this is too much information to process, maybe the next league after bestiary ends is a better idea? I don't think it should be too much of a problem. It would help a lot if you were able to hang out in discord with him or something to answer questions. If you need poe currency cheap in game, you can choose u4gm.com sale cheap poe currency.
"That's a league mechanic, just pick up the coins the chest drops but don't worry too much about it now."
The basics when leveling is the biggest barrier to entry for this game IMO. So making sure he's getting life and damage in tree. Looking for 3/4L items early. The basics of how active/support/aura/etc. gems work. There's a lot of knowledge experienced players have that they take for granted. Basically if you can be an interactive wiki for your friend it should help a ton.
First of all he would lose the opportunity to play flashback which is a pretty rare event. It is also going to be a more solitary experience since most people will be playing flashback on streams, reddit etc which makes participation more enjoyable.
I don't think playing standard is a good idea for anyone starting POE at this point. Nobody talks about it, there are no league challenges to guide you (most of them are aimed at beginners after all) and you would have to get all the way to maps and zana to see what league mechanics are if you even stay interested until then in the first place. And anyway what part of flashback mods is going to be "overwhelming" to new players exactly? Every mod is basically "more monsters" or "faster monsters" or a combination of those. You don't have to be an expert in the game to participate.