Hi! (Genuine question!)

So i have been together with my girlfriend since 2016 and at the same time i met my best friend, but the past 2 years me and my girlfriend has open up our relationship and we are still going strong, no jealousy everything is perfect she has hooked up with guys of internet and im taking it slow and just sexting with girls. But lately me and my girl has been talking about the idea of her sleeping with my best friend cuz she s really into him, and me and best friend are super close we tell each other like 95% of everything and he knows we are in an open relationship. How would anyone here approach this? How would yall tell him?

Thx!