I mean, alcohol is always a good start. Depending on your friend's age, either start drinking n invite her to join you, n make her part of the group, or get her drunk then get him involved. Either way, get her seeing him more like a man instead of her little boy's friend.
Then when you're all having a good time find a way to casually drop how big his dick is into conversation, but don't be like "yeah so anyway mom, he has a huge dick" be more like "fuck you man 😂 just cause you got the biggest dick in our group, doesn't mean you gotta wave it around like that"
Just subtle enough that even if she doesn't realise it, she'll start thinking of him as being a Man with a COCK, and not a boy with a penis.
If you take it slow, she should continue thinking about his dick over time especially if you keep letting slip how intimidating it is, but if she doesn't like you mentioning it, Act like it was an accident, but don't stop. Just keep doing it then saying sorry. Eventually she'll give up trying to stop you
As for him, just because he's got a big dick, doesn't mean he'll want to stuff your mom like a thanksgiving turkey. So you gonna see what his views are in sex. He might be the type who only makes love, or only like tight teen girls, or even be too much of a friend to want to hurt you.
Or shit, he might already be making your mom his little white cum slut behind your back, n that's why sex with your dad has gotten n so unsatisfying for her.
A good option is to drunkenly tell him how he's such a good friend, you'd let him fuck your daughter if you had one, then laugh n drunkenly mention how since you don't have a daughter, he's welcome to fuck your mom, then be like "naww naww man, ah fuckin mean it man. You deserve the best out of life, n my mom is the fucking best. I wouldn't even give a fuck if you....gave her a fuck 😂😂😂😂 just don't go giving me no new siblings or shit, just return the favour one day 😂 besides, doubt she could handle that monster you got anyway. Shit, I'd bet money on it, legit fuxkin money.....Where's my god damn wallet? You sat on it? "
Not word for word maybe, but that kind of "drunk" confession. Then if he asks when you sober up, admit he's your best bro, n you really wouldn't hold it against him if he went after her, as long as he didn't lie to your face about it or keep it a secret.
But as always, go into this with your eyes wide open fully aware of what failure could cost you, and that if it happens and you don't come back here with details, that I'll be fucking pissed 😂