You may want to remind your daughter that gossip is rarely an accurate way to get information about somebody. Until she talks to her friend, she may not know what, if anything, is really the problem.
Also, I know my family is always so busy during summer days. Between chores, errands and family field trips, my boys can go days, sometimes weeks, without seeing or talking to their school friends. Perhaps your daughter's friend is equally busy.
In the meantime, encourage your daughter to explore new activities and new friends. Yes, it's disappointing to not see one's BFF, but it's also a golden opportunity to explore new things. Summer theme: NEW ADVENTURES!
Good luck!