My motto when it comes to dating is.
Figure out what the bare minimum amount of effort you need from the other person is. Tell them what that is, if they don’t meet that minimum, move on.
Just text him “where you been?” Let him know you want more consistent contact. If nothing changes, end things. Maybe he thought the contact was enough, maybe this, maybe that but, why waste time thinking about it.
I think it’s really ironic that your friend needs to figure out a solution to a relationship problem about always having figuring out the solution to all the relationship problems lol.
But seriously, it kind of sounds like this is your friends “fault”. From what you wrote, she’s been like this from the start and your friend entered the relationship hoping to “fix” her, which is kind of unfair on his part.