That question has featured in several recent stories of people who have given up on Christianity. ‘There used to be miracles but they don’t happen anymore’ they say. The implicit idea being that they never really happened in the first place.
Christianity has long harboured those who don’t believe in the miraculous at all and those who mostly don’t alongside those who fervently do. What seems to be happening now is not just a shifting of camp within Christianity but a leaving of it altogether.
There have always been those inside the Christian faith who have sought to downplay miracles to make it easier for people to believe. Yet believing in miracles has not disappeared and I doubt that for many that would the biggest objection. Yet even if it was the simplest explanation is to provide some stories and ask people to judge for themselves.
Suggest they read this and this or this or this. I could keep doing or this for a while as these stories with verification are not as hard to find as all that. In fact they’re not really hard to find at all. Then when you’ve shared all the stories, ask them what they think. It might not prove to them that God exists, but it might also suggest to them that unlikely sounding or even impossible sounding events do occur and often they occur in connection with prayer. Which should give them something to think about.