It depends on what you mean by "english."
Here in the states it's a catch-all term that means hunt seat, dressage, etc.
The biggest difference is the saddle. Western saddles have very deep seats with a high cantle and a big horn on the pommel. They have fenders, which are smooth wide lengths of leather, instead of strap-like stirrup leathers.
If you're used to riding hunt seat, the stirrups on a western saddle will be much longer than you're used to. You may or may not ride the horse on a contact - many western horses "neck rein" which means they turn simply by the touch of a rein on one side of their necks.
I've been told it is easier to switch from english to western than the other way around. I don't know if that's actually true.
Many people in my area think they ride western when they are simply slumped in a western saddle and not riding at all.
So of course those people, when they no longer have a saddle to depend upon for balance, find it very difficult. True western riders have a deep balanced seat much like dressage riders, only they don't use as much contact. I've known riders to feel very lost when they suddenly had what felt like no contact on the bit.
Sometimes here western horses are trained to different aids as well.
What part is your friend having trouble with?
