Helping a friend with immigrations

I have a friend who wants me to fill out an immigrations form (can’t remember the actual name of it) to basically say her relationship with her partner is legit. From what I’ve witnessed, the relationship IS real, but I don’t think it will last.

I’ve known her for less than 6 months but I feel like I should help her and just do it. The problem is, I have to attach a photocopy of my passport and give it to her. I don’t feel comfortable giving her my passport copy and I don’t want to be questioned by immigrations about their relationship.

What would you do?