I need advise from a woman on discussing something with my girlfriend?

Be sensitive. He was probably the one for her. She just really misses him. You might not be doing enough things for her to start forgetting her favorite love. Send her flowers and/or chocolates and be romantic, but not too clingy, unless she wants you too be. She also might have a lot of feelings torward him that she bolled up and needs someone to talk to ASAP. be that person that she lets her emotions out to.