They probably know a lot of this.
They may not know it's a problem, or if they do know, then they probably don't know what to do about it.
So that's how I'd go about this. Rather than saying they need to expand their support network, I'd ask them what stops them from expanding their range of friends, support and opportunities for connection?
Means you're not pressuring them or saying you know what they need better than they do. It's asking about them, what they think and feel.