How can I check who my friend is messaging on WhatsApp for free?

I’m worried about my friend who has been acting out of character lately, and I suspect they might be communicating with someone who is a bad influence. Is there a free method to see who they are messaging on WhatsApp without buying expensive monitoring software? I just want to ensure they’re safe and not getting involved with harmful individuals, but I’m unsure if this is technically possible without their knowledge.