My only concern was if this friend would be offering too much, and potentially putting themselves in financial trouble should they lose their job, etc at the wrong time. Since you've stated that's not the case, it can't hurt to go for it as long as your both agree on the financial side of things.

You've got a set of fixed costs that your friend will incur, from printing, to whatever fee the site has where you'll do your showing. It may be a fixed fee for space, or a percentage of sales, or some combination of both.

You'll probably feel guilty while your friend has not recouped their losses/investment. You may suggest they take the majority of whatever revenue there is until they've recovered their costs (80% - 100%). There's a range there because your friend may also feel guilty taking all your revenue for your art, and wants to ensure you see atleast some revenue, even before they break even, so you're not completely discouraged. If this friend really is super wealthy, and the upfront costs are nothing to them, perhaps they'll suggest something more along the lines of 50-50%. Maybe they'll even suggest you take nearly everything. I'm mainly pointing out that the split isn't necessarily a fixed amount, but may vary until the costs are recovered.

Once you've recovered the initial investment, you'll want another set of percentages for some sort of fair split with your friend. I have no idea what that split may be. Maybe they'll just be happy having broken even and helped a friend, and you'll see 100% at that point. Having used the word 'investment' though, perhaps they'd like a cut.

To avoid potential fights, should you get extremely lucky, you may also want to agree upon a third split. There is a chance that someone may not buy a print you brought to this showing, but asks about other prints. Perhaps whoever it is acquires art for, say, a hotel chain. They order 100 copies of your ducks at sunset picture at 30"x20". Maybe it becomes one of the stock IKEA prints and they want rights to the image rather than buying prints. Who knows. This sale is a direct result of your friends investment, by bringing you exposure at the showing. But it's both a much bigger sale, and a sale for which he hasn't fronted your printing costs. I'd expect you to walk away with the lions share of a deal like this.