Tell her the truth and do my best to get it cleaned. It's weired that all the respondents are males....hahah!

What would you do if you got your best friend's dress dirty? I would lend her my best dress, as I'm sure it would be cleaner than the one she was wearing before.

I would tell him to stop wearing dresses! It's very unmanly!

I would lend her one of mine hoping that she wouldn't need hers back before I finished having it cleaned. It really depends on the situation. Was she wearing it at the time or had you borrowed it to wear?