It could mean one of many different things. Third party dream interpretations tend to be more difficult and overall less accurate than if it had been your dream we were being asked to interpret. It is only what these symbols specifically mean to your girlfriend's dreaming mind that counts in the end.
Milk is a symbol of learning, knowledge, plenty, fertility and immortality, while there is an old wife's tale that over heating and spilling milk is regarded as being an omen of bad luck.
Or it could alternately be a play on words.
"Crying over spilt milk"
Definition: cry or complain about something that has already happened
Explanation: Used when saying that something belongs in the past and the person should forget about it.