it's not the bit bothering the horse, it is the hands at the end of the reins. sounds like your friend is afraid of her horse, or is taking out her frustrations on the horse. people do that without knowing they are doing it. you might ask your friend if there is something bothering her when she gets mad at the horse next time.
If your friend rides so infrequently that her horse is too fat, there's a chance that the horse needs a training review. I'm sure that she remembers that the horse used to obey commands, and she's frustrated that he doesn't behave like he used to. A horse isn't a big furry bicycle, that can sit in the barn for a year and and perform the same each time it is pulled out. He's more like a kid who's enjoyed a long break from school. He needs to review some of the simpler things, and he needs to not be beat when he gets some of the answers wrong.
Retraining can be fun. For example, if the horse has a problem with stopping, work on it in steps. Definately don't go faster than a walk, and don't yank or pull if you don't get the results you want. Decide what cue you will use for "stop", and only use that cue, no matter what. Reward him for any approximation of success. If he slows down for the cue, reward him. Then, if he offers to pause for a moment, reward him more. It's just like a game of "warmer/colder" that you played as a kid. Reward him if he gets close to the answer, and continue the cue if he get farther away. Eventually, something inside his head will "click" and he will get more consistant. It is an amazing feeling when you see that the horse understands and wants to do what you ask!
You can improvise and use every opportunity to establish herd leadership. For example, the other day when I needed to move my 4 yr old Qh out of the way--no halter, no whip available-- I pushed him backwards on his neck with his head on my my shoulder. I repeated "back, back, back" and moved him about 6 steps to the rear. This also gave him an opportunity to do something good and get verbal praise for it.
Honestly, though, this is more of an exercise for us, instead of your friend, because if he/she isn't asking for the advice, he/she will never appreciate it.
I know it's hard if you don't have your own horse, but having been your age and gone through the same thing, I'm going to give you a piece of advice. Stay away. When there is a 'problem situation', it's a good one to stay away from, even if it means a free ride. If you can't bring yourself to find another horse to ride, the best thing is to say nothing and just enjoy the ride however much you can. Or find a different horse to ride where you see more eye to eye with the owner and there isn't a difference of opinion brewing.
When you start telling someone they need to change how they ride, feed or handle their horse, they do not like it, and most of the time, it only means you and her will not be friends any more, and nothing will change about how the horse is done.
It is a rare situation when you can get anyone to change how they do their horses. What normally happens? They get an attitude and they just get mad. If you ride better than they they are going to get mad, if you try to tell them to change something they like doing...ditto. Too...most people, they don't think of their horse as an athlete that needs to be conditioned gradually and then kept in work and not allowed to get so fat. They like having a horse that's rreally fat, to them they think that looks healthy. They think more of their horse as like an all-terrain vehicle. When you feel like riding, turn the ignition and drive it.
You aren't going to change that whole way of thinking.
Besides...you know...the horse could even be tossing its head and acting funny with you because you use your hands on the rein harder than the owner. It can be from many different things - their teeth, they just are frisky because they haven't been ridden in a while, or a huge difference in how you and her ride. And it would be up to an expert to really say, whether your way is 'right' or her way is 'right'.
The easiest way I found as a young person was to get a job riding horses for someone who was a good trainer and would be doing things in a more systematic way. Or, get your own horse. Or, take riding lessons from a good trainer.
If you happen to luck onto someone who has the exact same ideas as you on how to train and handle and feed horses, thank the Lord and do your best to keep on ntheir good side!