Younger guy [18] interested in my girlfriend [21] [cuckold’s perspective]

So, the guy straight up told me that she has a nice ass.
He never had a girlfriend, so me and his older brother have tried to motivate him to talk to girls, up his game etc. We helped him text, go on dates, and it mostly worked, however it never moved on to a relationship. Just bad luck, I guess. By an accident, he has sent us a dick pic in the groupchat. It stood there for solid 5 minutes. Later he recognized that he shared it together with multiple photos and deleted it.
And boy was I shocked. The picture was showing his penis, next to a book in his granddads library. Judging by the size of the book, which I have in fact read and borrowed from them, he must've been around 23-24 (9+ inches) cm long. It had a decent amount of girth to it too.
So as it was a bizarre thing in my day, I told it to my girlfriend, after a long day of studying. After telling her his supposed size, she told me: "Oh baby, I mean, at least you're not SMALL small.", and joked about his large size. Her size preference is large girth, and over average length. I figured that by her dildo shapes. She knows, and supports my cuckold fantasy, just hasn't been able to act on it due to her own anxiety, body dysmorphia etc.
For context, I am around average at 16 cm (6.3 inches). She likes my size, however she would prefer it if we had sex more often, and if I could last longer. For even more context, his older brother, is 18 cm (7 inches). That's what shocked me, that big difference of more than two inches between the two brothers.
A few days later, yesterday, I found out that they started following each other on instagram, and that they are sending each other memes. He is talking very often about my girlfriend, even in the groupchat. When I talked to him in person, he commented some potential girls to date, but told me that they definitely don't come close to my girlfriend's thick bottoms. I thanked him, but he continued to talk about how good my girlfriend looks.
So, is it a good idea to let the situation undwindle? He could get his first experience, and I could finally live out my fantasy. Also he's still in puberty, and I've heard that his libido is extremely high, as he has urges to orgasm multiple times daily. I think that could be a good experience for my girlfriend.
Or should I separate them, make a stop for this situation? I don't know how would his older brother react, and it's a very weird question to ask beforehand. We are very good friends.

I wrote the events a bit longer and detailed, considering some might like it. I think it also explains the situation a lot better

TLDR:
Hung younger guy and my girlfriend are interested in each other. It would be his first experience, and my fantasy come true. He is a friend.