Hello, My Friends Young Daughter Had A Huge Blister Come

hello, my friends young daughter had a huge blister come up on a breast ...they went to Dr....he has no idea what it could be so he cut a huge square out of her breast where it was. leaving a very huge open area where it was and sent them home.....she said they could not see any bite marks...but now its draining ...what could cause this? shes afraid it maybe cancer