She CAN afford a pro, she just doesnt think she needs one now, until 10, 20 years from now she still hates you because you ruined her wedding...
they can have a destination wedding fly from MA to Puerto Rico but they don't have the money for a pro? They do, they might have to cut budget somewhere else, but I'd not touch this with a 10 foot pole. Go as a guest, sure, and bring your camera, but insist they hire a pro.
This is just my opinion and I'm speaking from experience.
Everyone always saves the photographer last, we get the least amount in their budget, but honestly, we are the most important part of a wedding, IMO.
In 20 years, no one is going to remember that their cake was dry, or that the decorations were not perfect, UNTIL they look at the crappy pictures, and hate you all over again....
Unless you are 100% confident and CHARGE what a starting wedding photographer is priced, don't do it.