I'm sorry you lost your friend. There is a 'catch' to Mark 11:23-24... believing in one's head and believing in one's heart are not the same, overcoming our flesh (that is, our sense-ruled logical mind) and believing the impossible regardless of what natural logic and sense knowledge tell us... this is what true faith is, and it can be difficult- actually, it is impossible, without tapping in to the power of the Spirit within. I have not arrived yet, but I have started the journey. Let God be true, and every man a liar...
My aunt passed away about a year ago from cancer. I was believing for her to recover, believing and confessing the truth of God's word found in Isaiah 53:4-5 that by His stripes... After she passed, I talked with my cousin, her daughter, and learned she had asked my cousin to stop praying for her recovery, that she had lived a good full life and was ready to go home. After learning this, I was at peace, I understood why she was not healed and as odd as it may sound I was happy for her, her prayer was answered and she is very happy where she is. I was reminded that death is not the end, but a new beginning, and for the believer a wonderful beginning indeed. I miss her, but I'm happy for her, and I'll see her again soon enough.
Jesus said to her, “I am the resurrection and the life. Whoever believes in me, though he die, yet shall he live, and everyone who lives and believes in me shall never die. Do you believe this?”
I believe.