If the printer supports IPP advertised via Bonjour (mDNS), with PDLs including PDF, then I think all that's missing is the Apple Raster (image/urf) PDL on the mDNS entry and the URF entry. The image/urf support doesn't have to be real, since it's proprietary CUPS on Linux won't support it but I haven't run into an issues yet, I think iOS uses it mostly as a marker and otherwise respects the pdl ordering. You might look at the Discovery app to see what the mDNS records its currently advertising are. The URF entry can encode a bunch of information (info here: https://wiki.debian.org/CUPSAirPrint), but I've gotten away with URF=DM3.

I believe many Bonjour printers are of the kind that you can create by plugging a USB printer into an Airport, and they require a driver on the client. Apple makes this pretty simple by automatically searching for the driver based on the usb_MFG and usb_MDL from the printer's IEEE 1284 Device ID. None of these drivers would work on iOS, given its a different OS on a different architecture. I think AirPrint was the first driverless printing standard, followed by the PWG standard, and since they used existing technologies I assume they wanted iPhone users to only see printers which would actually work.

Bonjour also supports PostScript printers, which would use a PPD file on the client to configure the UI options and generate a PostScript preamble (this also requires generating a PostScript file from a PDF on the client -- a somewhat expensive operation). I don't know much about this apart from the couple of times its mentioned in the spec: http://devimages.apple.com/opensource/BonjourPrinting.pdf.

The practical solution is to have your friend configure a print server and advertise AirPrint mDNS on their network -- besides printing from my own iPhone, having friends be able to print to my decades-old but mighty HP LaserJet just by hopping on WiFi has been great.