I Hate Goodbyes [I Don't Like Goodbyes]

I dunno how to comfort my girlfriend when she has to say good bye to her 18 yr old daughter.

See, her daughter lives with her dad, and when she visits her daughter everything is fine up until the good bye part. She cries.

How can I comfort her? I don't have any chilfren myself, and this is the first I've dated someone with a child..