I also have Vonage and have no noticeable problems. But your friend needs to know that if he switches to D*, he will either need to contact his local baby bell or keep paying Comcast for the VoIP service. It may seem obvious, but I have heard sillier situations
And his Comcast bill will go up a little by dropping the TV portion of the service, as he will no longer be fully bundled. It may ultimately be cheaper for him to drop TV and Phone, keep high speed internet, and sign up for a service such as Vonage. I know in our area, Comcast Digital Voice is being offered at an introductory rate of $36/month (when bundled) and then goes up to about $40 after intro period is over. Vonage is $25/month, no if's, and's or but's. And here's another money saving trick with Comcast...downgrade to lowest possible TV service, keep the Internet service, and your bill will actually be lower than Internet alone by about $1-$3/month.