Hey, if a QH that's built like a tank can do hunt seat, so can a western-type Arabain (and yes, they do exist!) It all depends on what the judges in the shows you'll do are looking for. If they want the stock-type look, from what you describe, they'll do well. If they want the Appendix QH or English-type look, they will probably have a harder time. Again, it all depends on what the judges are looking for.
Who knows, maybe your daughter will get along so well with this horse that she'll decide to dabble in Western as well. I've done it all with my Morab gelding (who's a 50/50 blend) and he looks good in all disciplines except saddleseat (not enough lift in the front). You're welcome to check out my photo album at Webshots or my webpage that's desperately in need of updating. Both links are in my sig below
Out of curiosity, did the current owners tell you what type Arab he is (Polish, Crabbet, etc)?
Please visit the Morab Horse Association & Register, Inc. Dare to be different-Ride a Morab!
Bailey & J in the 2004 Midwest Horse Fair Parade of Breeds. He was 4 years old :)