Adding lower gears to a nine speed Ultegra road bike with triple ring
I’m not sure which version of an Ultegra derailleur you’ve got, but if it’s the 6500, it’s only rated for a 27-tooth cog and 37 teeth of wrap. You’re probably at 31 teeth of wrap. 

It might be more productive to put on smaller chainrings. Depending on what she’s got now, that’ll get her an extra gear or two.