WWYD?

So I am watching my friends little girl this weekend. She is 6, around the same size as my oldest height wise (48inches) and is about 60lbs. Her mom dropped off her harmony backless booster for her to use. Do I put her in it in my car, or do I put her in our HHB and put my oldest in the backless booster? Or just leave it? My oldest weighs less than her by about 6 pounds. My only other option would be to make my almost 5yr old (will be 5 on the 16th of this month) squeeze in the scerena (but she's around 38-39lbs) and take the booster from my husbands car and put her in that. (to do the three across in the back row seat of my van) My friends little girl is used to riding backless though. Is there really a huge difference (like am I going to have to listen to her complain the entire weekend about it?)

Right now I have:
mine almost 7 turbo booster
mine almost 5 apex 65
hers 6 harmony literider
mine 3 nautilus
mine 20 months rear-facing in EFTA

I usually have the nautilus and the turbo in the back and the apex and EFTA in the front captains seats. I'd have to pull the Nautilus totally and move my 3yr old (who weighs right at 39-40lbs) in the Apex and put the scenera in the middle position between the two boosters. It would either be the big kid booster from my husbands car or the harmony her mom dropped off. HELP!

I would let her use her Harmony. IME it positions the belt extremely well...quite possibly better than your Big Kid would. It's also very easy to buckle in a 3 across.

Using the harmony would be the easiest choice for me, b/c i can fit it between the turbo and the nautilus, so I wouldn't have to move anything. I just want to make sure that is my best option for her. I'm pretty anal about it I guess. Another friend wanted to take my oldest DD somewhere yesterday and I said NO b/c she has too much junk in her car. Like seriously, the car is covered in stuff, just random pieces of junk EXCEPT where her daughters booster seat is, and the drivers seat. I told her that I don't like how many projectile objects are in her car, and she said "its not really THAT bad" Um, yes it is...:thumbsdown:

Will she have head support in the seating position where she would be? If so, if it were me, I probably would just use the no-back booster since she is already familiar with it and it was what her mother provided for her. If she was smaller/younger though, I probably would be trying to find a way to get her back in the high back booster.

I would feel fine about a 60-pound child using a backless booster as long as there is head support behind her head.

yes, there is a head rest behind where I would be putting the booster. Thanks for the input guys!!!