Is this woman considered a betrayer of her husband, will God forgive her sin, and what advice can be offered to her?

Your friend erred by becoming involved with her former teacher, then by not cutting ties with him when he began to express his love for her. She also erred by giving him her home address and enabling him to enter, which is a deception of her husband. She must repent by feeling remorse and resolving not to return to such an act, and by completely cutting off contact with this man. She should conceal herself and not inform her husband or anyone else about what happened, for the Prophet, peace and blessings be upon him, said: "Avoid this filth which Allah Almighty has forbidden. So, whoever is afflicted by it, let him conceal himself with the concealment of Allah Almighty."