Re-tapping lands at Regular
From the JAR (my emphasis): Magic is a complicated game. While we can’t prevent players from making mistakes, we should do our best to fix situations as fairly as possible. It is important that players know that when help is needed or something goes wrong, calling a judge is the best course of action. Our solutions should focus on educating the players and keeping the game going rather than worrying about the impact on the game.
You have a lot of leeway here, and it really depends on you and your players what is the best course of action. At Competitive, this is easy: A made a mistake by tapping unfavorable but legal lands, nothing to see here, move on. At Regular… it's not that easy. Whatever you do (rewind or not) either A or B is going to be unhappy. The question now is who you want to provide customer service to, and this holds the danger of favoritism.
Since nothing illegal happened, I myself am in general very inclined to do nothing, and explain to A that he/she needs to plan ahead better. After all, is A has the Lightning Strike in hand but chooses to first cast another spell, he/she can't be completely unknowing of some basic strategy, right?
There might be a very specific situation where I would rewind, but this seems extremele unlikely. Customer service is great, disadvantaging other players is not cool.
However, remember that at Regular, if both players agree almost every ruling is correct. So if B has no obligations to rewinding (because he/she also knows to be the superior player and wants to do new player A a favor), that's perfectly fine! Just be careful not to put too much moral obligations upon B, it should be their own and free decision.