Most established wedding planners have a list of vendors they prefer to work with. Before you sign a contract, it's smart to stipulate that you can use vendors outside your planner's network. By doing a little bit of research and price comparison, you can often keep prices and quality in check.
While most planners are on the up-and-up, it's possible that she's referring only florists, photographers, cake-makers and other vendors that give her a monetary incentive to do so. On the flip side, she could be referring less-than-qualified friends who are trying to build their reputations in the industry. Chances are that your wedding planner is the utmost professional who won't mind if you keep your eyes and your options open.