just say no. weddings are ENORMOUSLY stressful and aren't worth the effort unless you're getting paid a good chunk of change.
There's a LOOOOONG thread on here recently of someone in a similar situation.
Defer your gf's friend to a professional photographer. Save yourself the stress and headache.
The cheapest clients are the most demanding and are the biggest PITA's.
The more people invest in wedding photography ($$$), the more likely they are to like their images and respect their photographer. I think it's called confirmation bias or something along those lines. They look for reasons to justify their purchase.
In contrast, doing a wedding for cheap or free doesn't require the client to invest in your photos. So there's no motivation for them to search for a justification of quality. Instead, they're going to be harsh critics, demanding and a pain in the ass. Dont believe, just ask anyone on here about the frustrating clients they dealt with until they moved out of the entry-level, budget wedding market.