First things first, the baby is not only too young/small to be ff'ing, the baby is nowhere near meeting the minimums of the seat to be ff'ing - 34" and 22 lbs are the requirement to use the AOE ff'ing. If baby's head is dropping forward rf'ing, I see no way possible that baby's head isn't falling forward even worse being ff'ing - the most reclined position can't be used when the seat is installed ff'ing, and 80% of the seat has to be touching the vehicle seatback, so quite often only the most upright position will work.
You'll likely need a towel that's tightly rolled to get a good recline. Definitely remove the base, it makes it much easier. The front of the cover on the 3in1's is attached my little elastic bungees. Unhook them from one side at the front, and it will allow you to see the belt path and reach it easily. The lap belt will need to be threaded in front of the buckle - as in towards baby's knees. The best method for tightening a lap belt with the 3in1, is to feed the tail of the lap belt up through the side of the cover, so that it's back through part of the belt path, and poking straight up the side. Then put pressure on the seat and pull on the tail of the lap belt and it should snug right up.
The rolled towel needs to be placed before you tighten the seat up, and you may need to expirement with a few different sized towels to get one that gives you the right amount of recline. Make sure you're pushing the towel or pool noodle right into the edge of the vehicle seat bite, and have it rolled up tightly. It needs to fit between the seatbelt and the buckle stalk and can't be any longer than that.
Hope that helps some. As for the seatbelts, if your friend is convinced that the outboard ones don't work, then install the seat in the center and have her take her car to the dealer to get the outboard ones looked at. It could be she doesn't understand how they work, but in the absence of a tech to tell for sure & if you're not certain, I'd just put the seat in the center & tell her to take it to a mechanic.
You should be able to get a perfectly good install removing the base and tightening the lap belt the way I described though. And I'm in agreement with the others - I'd highly encourage her getting another seat. The 3in1 seats are a huge pain in the butt and are much more money than they're worth considering that they're outgrown so young and will have to be replaced anyways. The harness is a pain to tighten, the top headrest position isn't usable in harness mode, and the seat is just a general PITA to install until you've done a zillion of them - and even then some vehicles are still a huge pain.
I really hate the 3in1 seats, yet they're easily the most popular around here. Probably 80% of parents buy them I'd guess... I've never seen a scenera at a check, never seen a triumph, have done a couple private checks with titan's and have seen a few britax's. Other than that, every.single.convertible has been a 3in1 (2 were the enspira, the rest were the AOE or EB.)