She's going to worry about friends, and it's good that she cares. Tell your daughter that she can demonstrate excellence and competitiveness in a fair context that will help her friend succeed too. Encourage girls not to be afraid to win. It can be a gift. And also encourage girls to talk together about it.
Many men fear female victory.
Some girls are so aware of that that they stop competing with boys. Many boys are uncomfortable with themselves and trying to figure out what it means to be a man. They think being a man means being faster, stronger, and smarter than women. Boys will say and do sexist things when they're confused. I would counsel a girl that they will be happier if they can find boys who respect and support their talents.