Lower gears for Shimano 9 speed?

I dealt with this exact problem on a mate's bike recently. Ummed and ahhed and went for an Acera derailleur and 11-34, he'll be fine on the hills now! It probably will be a bit gappy at points, but that's a much better compromise than walking up hills. Could have gone Sora derailleur but think the shadow design on the Acera is an improvement.

I'm not sure if the Tiagra 4500 mech will run smoothly with a 30t cassette (Shimano are conservative with their compatibility figures) but if you fit the cassette and wind the b-screw out far enough then you'll soon find out if it shifts cleanly.

The simplest alternative is to get hold of a Sora 3000 series rear derailleur, which will comfortably handle up to 32t cassette. Shimano says the medium cage GS will take 34t.

I run 11-30t with 50/34 chainrings with a Sora derailleur and don't find any big jumps. When you're in the 34 ring on a steep pitch reaching for the next gear down the 3 or 4 tooth jump in the bottom gears is a godsend, not a problem!

You might need to add a couple of extra links in your chain. Park Tool show how to calculate this correctly.

You would need to check if the rear mech is a ‘long arm’ type and could cope with, say, a 30T sprocket. Another way of getting lower gears in your friend’s situation is to fit an MTB cassette - say 11-34 (and new chain) - and an MTB rear mech. 9 speed stuff is starting to get trickier to get hold of, but something like a Deore M591 or M592 are the standard rear derailleur options. (If interested just check whether a barrel adjuster is already fitted or if an in-line one is required.) 

I have 9 speed Shimano although mine is a Sora rather than Tiagra groupset. Mine came with a 11-32 cassette and a FSA Omega sub-compact 32/48 chainset. If your friend wants to stick with their existing compact chainset then going for a wider cassette could definitely work. I find the gaps between gears okay although the jumps at the bottom end do start to become more pronounced.