My friend has Elder Sign, and was wondering if Arkham Horror was too alike to it - she knows I love Arkham Horror a ton, but she didn't want to get essentially the same game.

Nope. ES and AH share the very same setting (both in terms of the Lovecrftian Mythology and the city of Arkham being in big troubles because an AO tries to awaken - I guess Arkham's coffee must not be so good, most of the times they return to sleep), they are both cooperative, and they share the same protagonist, but they are two totally different games.

Arkham Horror is a not-so-light strategy card-driven game, while Elder Sign is quite a lighter dice game that implements in a lovecraftian environment the mechanics originally developed by Reiner Knizia for Risk! Express. Arkham usually lasts quite some time, while Elder Sign can be played very quickly, under 30 minutes max. Arkham is more difficult to master and even when you mastered it, it's more difficult to win, while ES is definitely an easier game. Easier not always means "bad", and this is the case: both games are extremely enjoyable

Okay thank you so much! I pretty much just copy and pasted what you said to her.

Lemme know if you need some more in-depth answer or your friend has other questions