There's a number of variables involved in this so can't guarantee either way what it'd be like for you/them (their connection to the DC, your connection to the DC, links between the two connections, how many players are on, time of day, etc).

I'm UK based and play on the Aether DC with a lot of American friends. Usually the lag is fine - yes, there is obviously some (typically I get a ping of around 110 - 160 ms during peak hours, lowest ever got was about 90-100ms with a dedicated VPN) but it's never really stopped me from playing/enjoying the game.

I'd say the main things affected are usually during battles - it's easy to get clipped thinking you were out of a mechanic hit-area when (due to lag) the game still has you registered within it - you soon learn how to slide-cast, move quickly, and it becomes less of an issue when you learn the fight mechanics well.

Easiest way - get them to create a toon on EU and you create one on US, spend a couple of hours playing each side and see which is better for you before properly starting out.