One way to support a loved one struggling with their mental health without overstepping boundaries is to simply listen. Let them talk about their feelings and concerns without judgment or trying to "fix" their problems. Offer reassurance and empathy, and let them know that you are there for them.
Another way to support a loved one is to encourage them to seek professional help if needed. Offer to help them find a therapist or counselor, and provide reassurance that there is no shame in seeking help for mental health issues. Respect their decisions and boundaries, while also gently reminding them that support is available.