Coilovers are undoubtedly better, but you're going to be spending an absolute minimum of £600, probably closer to £1k, rather than £200 for a set of springs.
Considering it's purely for aesthetics, I personally couldn't justify that extra cost if I was in your position. If she will be doing lots of track time or hard driving, then that's a different matter.
However, if she wants to go super low while still being able to tackle even the biggest speedbumps, then air would be the best option - but that's even more expensive.