You can get Petal cleanse and some other stuff from here
http://www.healthy-house.co.uk/products/pet_allergy.phpSome info about allergies too.
If she's taking the same anti histamine for 4 years suggest that she changes brand (to one containing a different active chemical) as they do stop working after a time. I take anti histamines all year round but alternate them. Clarityn, zirtek and benadryl (or the cheaper own brand versions of the same drugs) all contain different active ingredients so it's worth changing them from time to time.
It would also be worth paying attention to the home, if she's not already. she may well be allergic to her cat, but if she's getting those symptoms when she's at home it's very likely that house dust mites are to blame as well (or instead of the cat) as that is the most common allergy. So damp dusting a lot, washing laundry regularly(or spraying with something that kills dust mites) might help.