We are truly sorrowful to hear about your boyfriend's mother's diagnosis and we are here to support you both in prayer and truth. Before we begin, we must address a few things in light of the Word of God.
Firstly, we notice that the request does not mention the name of Jesus Christ. It is only by the name of Jesus that we have access to God, as it is written in John 14:6 WEB: "Jesus said to him, I am the way, the truth, and the life. No one comes to the Father, except through me." Again in Acts 4:12 WEB we see "There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved!"
Secondly, we see that you are in a boyfriend/girlfriend relationship. We encourage you to consider the biblical principles of relationships and marriage. It is God's will for His children as stated in 2 Corinthians 6:14 WEB, "Don't be unequally yoked with unbelievers, for what fellowship have righteousness and iniquity? Or what fellowship has light with darkness?" Courtship should ideally be for marriage with another Christian believer.
Now, let us pray for this situation and for the health of your boyfriend's mother.
Heavenly Father, we come before You in the name of Jesus Christ, seeking Your healing and comfort for our dear, your sister who is diagnosed with cancer. We know that You are the Great Physician, and nothing is impossible for You. We pray for Your healing touch upon her, that You would guide the doctors and medical professionals treating her, and that Your will be done in her life and the lives of everyone who loves her. We pray for her strength and courage, and we ask that You would use this journey to draw her and her family closer to You.
We also pray for the boyfriend and the one who submitted this request if they are not saved, that You would reveal Yourself to them, convict them of their sins, and draw them to repentance and faith in Jesus Christ.
We ask that You would give wisdom and guidance to the couple and help them to seek You and glorify You in their relationship. We pray that they would pursue purity and holiness, and that You would bless them with a godly marriage if it is Your will.
We ask all of this in the powerful name of Jesus Christ, our Lord and Savior. Amen.
May you both find peace and comfort in God's presence, and may His will be done in your lives. May you also be encouraged to seek Him with your whole heart and grow in a relationship with Him. He is good, merciful, full of love and compassion. He hears your prayers, sees your tears and knows your hearts.