Can I 'upgrade' a 9 speed Tiagra RD to Sora to lower my gears?
You have several options depending what is important to her, I am presuming cost is a factor.
11-34 cassette with new derailleur and chain (given cassettes and chains are consumable) is cost effective, but it does mean the jump between gear changes increases.  If she does not need the higher gears then there are ways to bodge a cassette of say 14-34 that would close the gap (at the risk ramps would not line up making shifting less smooth)
Decreasing the chain rings from 30, as commonly suggested, means to to get the same ratio as 30/34 would require a 23/26 (reality 22 or 24)  chainring, which almost certainly won't work with the existing derailleur and frame, and if it did, would also require new cranks, i.e. not cost effective.
A hybrid solution of a smaller chainring and larger, but not as large as 34 cassette, adds the cost of a chain ring but reduces the jump between gears, but allows you to tert some things before spending money that may not be needed  This could be done in steps-try say 11-30 cassette with existing derailleur (see comments - pushing it, but should work acceptably), if that works, but low gear is still to high, smaller chainring. If shifting is unacceptable, upgrade derailleur.
A final option would be go to a 10 or 11 Speed rear. If you are already upgrading the derailleur, cassette and chain, it adds just the cost of a shifter (and cable inner).  10 and 11 speed will play acceptably with a 9speed front end and the wide range cassette means front shifting is reduced anyway.  Obviously the decision to do this needs to be done before purchase.