You ought to tell her straight up. If she disagrees with you, show her (literally SHOW her) how much better off your horse is. If she still disagrees, she isn't much of a friend to you or to her horse.
Could the friend be a little insecure about her riding technique and/or skill? For me, if she feels that she needs all that equipment to "control" the gelding, that he's not a good match for her. Dressage horses are acutely trained... you cannot expect to pick up a pasture puff one day and say: "Now do as ____ did at the Olympics." Time. Training. Patience...
I have been in your shoes regarding this situation, and for me it did not end lightly. (On multiple occasions.) Suggest having your friend try riding without such a tight ensemble on the horse. How does he move? How does he respond? And sadly, there are such people in the world who don't have a clue what they're doing or what they want from their horse.... :/
Standardbred and Radiowaves nailed it for me. I'm 53, and sonny's 13. I very much look forward to growing old with sonny. I know the days when him and I both spend our time sittin' in the pasture are coming. And I can't think of a better friend (my wife excepted of course) to spend my time with. But our bond is strong because Sonny trusts me and knows I will take care of him. NOT mistreat him. You should talk to her trainer first, and probably both of you talk to her. But you or her trainer definitely need to talk to her. That is just plain and simply not the way to treat these magnificent animals. We're lucky to have time with them and they should be treated with respect and dignity. It's not right, and you know it!
I don't know about approaching said trainer with this.Any decent trainer that has seen the get up she has been using & her riding, would have said something a while ago.She obviously isn't learning anything valuable from said "trainer"
Many good and useful things mentioned on this thread. Since we're all pretty much in agreement that the good horse needs better treatment, how about this idea:
Ask to videotape your friend riding her (?, leased?) horse, so that you can both watch and she can give you some riding pointers. Perhaps she doesn't realize what she is doing.
I guess if she sees herself and is pleased then no words will persuade her otherwise.