I've been in this exact situation. You definitely need to say something...in love. I think it is important to have scripture to back up whatever you say.

Job 31:1 says "I made a covenant with my eyes not to look lustfully at a woman".
Matthew 18:9 "And if your eye causes you to sin, gouge it out and throw it away. It is better for you to enter life with one eye than to have two eyes and be thrown into the fire of hell."

There is also a GREAT book called "Sex Isn't the Problem, Lust Is". My husband has it on CD and listens to it regularly to remember what God says about this issue.

I will be praying for your situation...that God will give you wisdom to know when and how to approach this. And, just so you know, it has NOTHING to do with you. Don't take on guilt or shame for not being pretty enough or thin enough or anything other lies that the enemy will try to tell you. In our case, we chose to tackle the issue together hand in hand with Jesus, and he is completely free.