The PCD is smaller on compact chainrings, due to the ring size, so cranks would need to be change also, the cheapest option is to buy a complete chainset with the smaller chainrings
the smallest standard rings i have ever come across are 51 and a 38 that fit a standard chain set.

you need the 130mm PCD size on a normal (53/39) Shimano chainset.
You need 110mm PCD if it is a compact chainset (50/34).

Don't worry about 8 or 9 or 10 speed descriptions as the chainrings are virtually the same.

Sorry if this gets a little technical but it does depend on a few factors. A sora rear derailleur can support cassettes within the range of 11 teeth and 27 teeth. So you could make sure that you have a cassette that has a larger ring of 27t and perhaps goes down to a 12 or 13 teeth for a easier ratio. This would be your best option as your girlfriend is already running a triple chainset. The smallest ring on the triple chainset should be small enough to provide a easy ride with a 27t ring at the rear. The small 30t ring at the front can be swapped for a 28t but this will max out your possible ratio difference at the front of 22t (50t - 28t). Hope this helps.

We do stock compatible chainrings from TA here: http://www.wiggle.co.uk/p/cycle/7/TA_135_PCD_Horus_11_Speed_Outer_Chainring/5360044470/

You should be fine with rings from TA, Truvativ, Race Face, FSA or any other Shimano compatible rings. You need to order the rings in the correct BCD and 4/5 arm configeration.

The Merida 901-18 uses a Sora front Derailleur which has a 16t capacity. This basically means that unless you swap your outer chainring the smallest inner ring you can use is 36t which unfortunately we do not stock. If you changed your outer ring to 50t though you could use a 34t inner ring as seen here: http://www.wiggle.co.uk/p/cycle/7/Shimano_110_PCD_FCR700_10_Speed_Inner_Chainring/5360038318/

This one should work:-
http://www.wiggle.co.uk/p/cycle/7/FSA_104_PCD_DH_Chainring/5360031667/#more

You can find a replacement under Chainsets Road but we do not stock the triple Gossamer chainset at this time. The closest replacements would be the Truvativ Rouleur GXP Triple and the Shimano Ultegra SL 6604 Triple chainsets at the links below:

http://www.wiggle.co.uk/p/cycle/7/Truvativ_Rouleur_GXP_Triple_Chainset/5360021061/

http://www.wiggle.co.uk/p/cycle/7/Shimano_Ultegra_SL_6604_HollowTech_II_Triple_Chainset/5360031343/

Unfortunately we have no plans to stock the 110PCD 52t chainring.

I am sorry but we do not stock these chainrings. You may be able to find a compatible ring amongst the TA range but you will need to know what speed the rings are and what PCD you require: http://www.wiggle.co.uk/mc/cycle/7/TA/Chainrings/