My sister lives in Scotland and is having major trouble growing coriander aka cilantro. Any tips for her please?

Question

I think was over watering at first, so stopped doing that… Grew pretty good, then would die.

Answer 1

Create good growing conditions: besides the water management, build soil organic matter with compost and use a complete organic fertilizer.

This article has some additional information:
https://www.gardeningknowhow.com/edible/herbs/cilantro/tips-for-growing-cilantro.htm

Even with ideal growing conditions, this is a short lived herb. Pruning cilantro frequently will help, but no matter how much you prune cilantro, it will still eventually bolt. Plant new seeds about every six weeks to keep a steady supply throughout the growing season.