THE PROBLEM WITH YOUR THEORY IS
HE WAS ALREADY FOLLOWING HER FOR BEFORE
SHE FOLLOWED HIM THOUGH...
SO WHAT WAS THE POINT OF FOLLOWING HIM?
the follow bots may be based off hash tags or they may search for something in her explore page that he may of popped up on, or in other numerous ways you can set bots up and thats why she followed him after he followed her.
Keep in mind, bots are blind and emotionless and that's why they are used so often as theyll do activities (follow, like, unfollow, etc.) in ways that a normal person wouldn't
example if her bot is set to follow accounts that have the hashtag #lambo and he posted a picture with the hashtag #lambo, the bot may of followed him.. without the bot knowing he followed her previously.
I ran bots for my vegas promoter accounts... crazy effective but instagram added limits to newer accounts that made it harder just a month or 2 into me doing this(this was years ago so maybe theirs a way around these limits now) example if your new account was new it could only like 20 pictures a hour or some ridiculous low amount so my accounts got banned a lot, or if i uploaded too many pictures in a day.. it just turned into too big of a hassle for me to continue it
this was around 2014 or so and the accounts were all new and maybe I did 2 many at once
maybe its effective just doing it on 1 account
edit found the updates:
"Updated 7/12/14 According to multiple people, but with no official word from Instagram, there is now a new limit restricting users to follow and unfollow no more than 20 people per hour."
and they made it so you could like 300+ people pictures a hour down to like 50 I think or some low shyt..
this usually only applied to newer accounts though
basically the people who botted before 2014 ate very well on instagram with almost no limits