What is the EXACT law about getting into a far lane when ping a police car that has someone pulled over?

The other day, my mom was going done a highway and I saw a trooper had someone pulled over with his lights on. I told her to get into the middle lane but she didn't until AFTER she ped him! I had warned her about the same thing the day before. She said she couldn't do it sooner because there was another car in the lane she needed to get over in. Well, she got pulled over this time. The trooper was quite angry and I don't blame him. He said she should have slowed down and then got over if she couldn't at that time. She could have hit him! She is 69 and I think he understood that older drivers aren't as "sharp" when it comes to quick responses like getting over in another lane and are really weary of getting hit. He didn't give her a ticket but a stern warning. After he left, she told me that never was a law years ago. I told her that I'm pretty sure it is a law since a lot of troopers have gotten hit by drivers not getting over. What is the law? We live in Michigan. THANKS!