Watch your boundaries.
The first rule of first-aid states "always make sure it is safe to approach". It means that we can't offer meaningful support to someone with a mental health problem unless we take steps to keep ourselves functioning.
Helping and supporting someone with a mental health problem can be a time consuming and draining endeavour. It uses up a lot of internal resources, and can leave you feeling overstretched and in need of support yourself. Within limits many Christians are willing to accept this as a part of their ministry to others. But we all have limits, and putting some boundaries in place before you begin to help can be really useful. Boundaries can be viewed as simple steps we take to protect ourselves from becoming too involved with those we are seeking to help to the point where we are no longer helpful.
They also serve to help the person to take some responsibility for working through their problems themselves. It does not do anyone any favours if you try to sort everything out for them and they remain passive when they do not need to.
So, for example, it is really useful to put time limits in place when supporting someone who is struggling with their mental health. If they come to talk things through with you, make sure you let them know that an hour, for example, is the maximum time you can spend with them in one session. It is not usually possible to solve people's problems by spending all day with them, unless there is a genuine crisis. It is ok to have limits, and also to put quite strict limits in place, especially if you are new to this sort of thing.
It is also important to remember that you are not indispensable. Share the load with others who are also able to help and support. Do not fall into the trap of believing that you are the only helper who can truly help properly, even if that is what you have been told by the person you are helping. This is rarely the case, and it is much better to share the helping so that no one becomes burnt out by too much exposure to strong emotions.