@purple premature ejaculation (PE) is really about an individual’s sense of control over ejaculation rather than an minimum amount of time before ejaculation. People can naturally have different levels of sensitivity to stimulation. In other words, some people will cum quickly & some people will take longer. Age can also be a factor: people in their teens & twenties with raging hormones & limited experience are more likely to ejaculate quickly. Often with time, practice, & more confidence in relationships, this will change. Other common factors are: levels of excitment, amount of foreplay, levels of anxiety, & length of time since last ejaculation. @ekoorb9 posted a great list of strategies for building greater control of ejaculation. A sex therapist can also be helpful.
All that said, PE is really only a problem when someone wants to last longer & feels they can’t. It’s possible that the your friend’s partner is sexually satisfied ejaculating after 30 seconds. If your friend is not satisified, then the conversation really needs to be about expressing her needs & being creative about how those might be met (rather than only focusing on fixing a problem he has). Hopefully that’s helpful! 🙂