It's risky because she may see right through that story and then you would be incongruent. You will have use your best judgment but I don't usually recommend lieing. Sometimes owning up to your own mistakes helps. It could raise your value or diminish. It's a very tricky situation. If you decided to tell her the friend thing, when you text her let her know that you just found out who actually sent that text and that he usually does that with other peoplese phones.