They may discover it was tampered with. It likely has a label or copy of the return form on it, with your info.

I would guess this is not the first time your friend has bought an expensive cream and replaced the contents.

If I were you, I'd stay away from that store for a long time. Her pic and yours could pop up by the employee time-clock.

And I'd find better friends who won't put you at risk of getting in trouble.