I'd go for the Decathlon suits too, just in there tonight and they look alright. But I'd go for a full length suit, sunburn on the back of the knees isn't nice! She might not get cold but being in the water for long periods of time might leave her quite tired, I know it does me, just through losing heat so a layer of neoprene is worth it rather than a lycra skin. Decathlon's suits are dead cheap anyway and aren't that much more expensive than just a skin.
If she's into a bit more serious snorkelling rather than just fishy watching while floating on the surface then a decent freediving/spearfishing suit is worth its weight in gold. Have a talk to Neil (a great guy) at Apnea
www.apnea.co.uk They are in St Helier on Jersey but they send suits for free anywhere in the UK. I've just bought a 5mm spearfishing suit from them made by French company Imersion... brilliant suit. Two hours in the water on Sunday and hardly noticed it.