I wouldn't say much. Your friend has undoubtedly noticed the difference between the two boys just as you have. And she's probably just waiting for you to sound smug or make an unfavorable comparison. If you think the child needs some kind of medical or educational attention he is not getting (for example, enrollment in a high-quality early intervention program) and clearly needs this kind of help, I would bring it up. Otherwise, I should think you would want to wait for her to bring it up and try to avoid comparisons as much as possible.
As you bring the two boys into contact with each other, there is a possibility that your son and your friend's son will have little disagreements. Having to cope with your son's greater competence may be frustrating to your friend's child, and fights over attention, toys, and positive responses from the adults may result. You and your friend will have to work to maintain a relationship if too much conflict over the boys develops.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.