the saddle should not make a difference to a horse...meaning it won't really know the difference between an English and Western saddle. Remember, to a horse, a saddle is really just another thing on their back...western or english they don't really care. I've never seen a horse that will ride perfect in a western but not in an english (unless it's the riders fault)
My horse, Sonny, was trained in Western pleasure and was mainly used as a trail horse...so no English what-so-ever. But I ride him English.
I still keep a western bridle on him? Why? I like the western bridles better....and I'll switch back and forth from an English Eggbutt to a Western Snaffle with Shanks.
I would think she'd have no problem with you riding her English...if she's in a western bridle it may take some time for her to get used to the noseband, but other than that I'd say she would do fine