Can I just say thank you (genuinely) for recognising the situation is good for your son, which obviously you are grateful for, but recognising it might be difficult for the girl.
I'm torn on the situation because all through primary (overseas, 2 years / 1 teacher & assistant per classroom), my daughter was that girl. She was always put next to the disruptive or SEN boys. Some were truly horrid to her, stealing her stuff, knocking her or just saying nasty things. She never complained because she understood why she was the one that was placed there as they were even worse with others but she suffered as she couldn't concentrate properly and it sometimes wore her down very badly.
However, over the years, she was routinely placed, on rotation, between two children. One had severe SEN due to a birth trauma. He was a lovely, gentle boy, but still couldn't barely read by year 6, he should have been in a special school, but there just weren't any. My daughter would help him with everything but make him do things she knew he was able to so he had a sense of achievement.
The other boy came from the most awful family situation you can imagine and he simply acted out against it. He hit other children in the face, broke things, threw equipment and for all intents and purposes seemed an absolute horror. He wasn't, he was just a very damaged little boy. When he was fighting in the playground, the teachers would call for my daughter as she would go and intervene and calm him down. She also helped him in his lessons when she could as he also had some learning difficulties. I think she was the only child he didn't deliberately hit. He once smacked her in the face, but that was by accident during one of her interventions. When she left, at the end of year 6, he wrote in her leaving card, "Thank you for being my friend, I will miss you". I still well up when I think of it and what it must have taken for him to write that. She honestly didn't see how much she had helped him, she was just doing what she does!