Can I show one friend a text that explains why my other friend is acting very differently?
Oh my, why is she acting so differently?
Her boyfriend died terribly
If your other friend did not consent to that text being shown, you should not do it. It is a breach of privacy and it is not right to share secrets.
But that's what friends are for and she needs to have us be there for her.
She might need you guys but she does not need to be betrayed right now when she needs to able to trust you. If she can't trust she might not talk to you and she will be all alone.