Allen, 27, known for songs like "Smile", "The fear" and "Not fair", advised Styles, 19, to enjoy fame as it can be short-lived, reports dailystar.co.uk.
The singer told band One Direction member Styles: "Look at you now, Harry - everybody wants a piece of you. Enjoy it while you can. In a few years no one will recognise you, like me."
Allen is making her comeback in the music world with an untitled album, which she hopes to release this year. Her last album "It's Not Me, It's You" came out in 2009.