If the proceeds from any images you sell are going to the fundraising effort, it's obviously unreasonable to ask your wife to pay. In these circumstances, if they insist on charging for her ticket, you might want to walk away. I would.
However, if you're waiving the shooting fee but selling the images for your own profit (it's not clear from your original post), then it's not unreasonable or unusual for the organizers to provide only one free ticket. At most of the fundraisers I've done, even the organizers pay for their tickets.
I'll shortly be doing a photo booth for a fundraiser "spring fair", and will have a separate table as an exhibitor as well. I'm paying the going rate for my exhibitor's table, because I'll be selling my own products for profit. Just because I'm donating all the photo booth proceeds to the charity, it doesn't mean they owe me a free table for my business activity.
Doing work on fundraising requires a certain amount of generosity. However, the ticket price for the dance you're covering is fairly high. If it were me, if I were donating the images to the cause, and if I absolutely needed an assistant to provide the coverage they'd asked for, I'd probably (and politely) withdraw my services, explaining that, while I was happy to give my time and images, I hadn't figured on donating a sizeable cash sum as well.
Personally, when I agree to do fundraisers, I always donate both my time and the proceeds from any sales. It's kind of an unspoken rule, at least in my circle of activity, that contributors don't profit from charity events. However, in the hypothetical case that I was selling the images for profit and the organizers asked me to pay for my assistant, I'd either cover the event alone, without an assistant, or withdraw on the basis that the cost of my assistant's ticket would wipe out any profit I could make from the event.
Either way, I hope you don't fall out with your friend over this. Life's too short.