I don't know anything about your friend's mental illness, so what I am about to say is a very general statement.
I do believe in mental illnesses, but I also know that many wrong, undisciplined mindsets and beliefs can also masquerade as mental illnesses. I know that some people who claim to be mentally ill are really EMOTIONALLY ill - in the sense that they have emotional issues that they need to deal with honestly and responsibly in order to get their head together.
I'll use myself as an example. I am Obsessive Compulsive (OCD). I've been medically diagnosed with that particular "mental illness," and I've successfully used prescription drugs for that condition in the past.
However, this condition only manifests itself in a life altering negative form when I am under a great deal of emotional stress. Only then do I occasionally need to take medication for it.
Actually I haven't taken any meds for it for at least 15 years. I've been under great stress in the past 15 years, several times - debilitating illness, major surgery (which was successful, so I'm no longer struggling with health issues, thank goodness), divorce, son in Iraq with an infantry unit for 18 months. Why didn't I need meds to control my mental illness during those times? How am I able now to control it in my everyday life?
All I know is that I worked through a lot of emotional issues, baggage, and damage, much of which I had given myself, though I could have easily blamed others and played the victim.
Part of this self-therapy was the reparation of my relationship with God, and applying Christian principles to my life.
I still have OCD. On occasion, I struggle with OCD tendencies. If they began to get a grip on my life again, I would go back to a doctor about it and get meds if necessary - because that would be the responsible thing to do, and God expects us to work WITH Him and to take responsibility for our actions.
I would also hunker down spiritually and re evaluate my relationship with God and be sure it was on track. I know that in the past, doing this was an integral part of my victory over the sometimes overwhelming urges of OCD.
God can and does heal us - as His overall will and plan provides. Faith is submitting our will to God and accepting His will in our lives. This doesn't mean that we are always healed. It doesn't mean that we are always healed or that we live a pain free life. It means that, as others have said, God promises that He will never forsake us and that He will never allow us to suffer more than we can bear - with Him at our side.
Some of my favorite verses on the topics of thought patterns, behavior and God's promises to His children are:
From Romans 5:
1Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ, 2through whom we have gained access by faith into this grace in which we now stand. And we rejoice in the hope of the glory of God. 3Not only so, but we also rejoice in our sufferings, because we know that suffering produces perseverance; 4perseverance, character; and character, hope. 5And hope does not disappoint us, because God has poured out his love into our hearts by the Holy Spirit, whom he has given us.
In this you greatly rejoice, though now for a little while you may have had to
suffer grief in all kinds of trials.
1 Peter 1:5-7
1 Peter 4:12, 13
12Dear friends, do not be surprised at the painful trial you are suffering, as though something strange were happening to you.
13But rejoice that you participate in the sufferings of Christ, so that you may be overjoyed when his glory is revealed.
Peace I leave with you; my
peace I give you. I do not give to you as the world gives. Do not let your hearts be troubled and do not be afraid.
John 14:26-28
"I have told you these things, so that in me you may have
peace. In this world you will have trouble. But take heart! I have overcome the world."
John 16:32-33
The mind of sinful man is death, but the mind controlled by the Spirit is life and
peace;
Romans 8:5-7
May the God of hope fill you with all joy and
peace as you trust in him, so that you may overflow with hope by the power of the Holy Spirit.
Romans 15:12-14
But the fruit of the Spirit is love, joy,
peace, patience, kindness, goodness, faithfulness,
Galatians 5:21-23
And the
peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus.
Philippians 4:6-8
No discipline seems pleasant at the time, but painful. Later on, however, it produces a harvest of righteousness and
peace for those who have been trained by it.
Hebrews 12:10-12
But the wisdom that comes from heaven is first of all pure; then
peace-loving, considerate, submissive, full of mercy and good fruit, impartial and sincere.
James 3:16-18
This is what I believe.