FWIW, I'm getting a kit to use when I ride with my adult son. He's capable of 20mph+ easily and I'm able to sustain 14-15 mph max. After some rides where he would go ahead and then have to wait for me, I realized it wasn't working well for both of us. I'd just approach your slower friend with the idea that you WANT him to be able to continue to ride with the group and that time takes it's toll on all of us, so an e-bike would allow him to continue riding with you all now and in the future. I guarantee you, he knows he's getting slower and he's probably afraid to bring it up because it might mean not riding with you guys and maybe he's not sure you'd be okay with a motor (some roadies are not). Make it more about wanting to include him and I bet he will be relieved!