Approaching research adviser with worries about project feasibility
Ultimately you are the judge but by the looks of it, I would consider 3 weeks as too early to decide if a task is feasible or not. I am assuming that you know the colleague who graduated just for 3 weeks. That is a short time to know someone and calibrate their opinions with your own understanding and capabilities. 

If the supervisor has given you the task despite the other colleague did not doing much on it there might be some weight in the task. I would suggest you do your own thorough investigation and give at least 3-4 more weeks trying to understand all aspects of the task at hand. Prepare a document and clearly mention what are the risks and opportunities in front of you, what are the related works surrounding the task.

At the same time, try to best gauge the intentions of involved folks, may be ask questions to other people around the lab, find experts online and write mails.

With this strategy, I think, at best you will lead to some groundbreaking ideas or findings and at worst you will learn a lesson in how to systemically conduct research, gauge opinions of others and strengthen the collaboration channels.

Hope this helps.