Your friend is wrong. The Sidekick plan only works for the Sidekick. Nothing else.
You need the Blackberry plan.
The only reason I could think of why you want to keep the Sidekick plan is because of the unlimited text messaging. If that's the case, you need the following plans..
http://www.t-mobile.com/shop/plans/d...7-3ec0885e2004
http://www.t-mobile.com/shop/plans/d...1-a45eaf224493