Hello and thanks very much for your question!
As long as she has no allergies to anything codeine related, I usually recommend a dextromethorphan product such as delsym or a combination with guaifenesin and dextromethorphan such as mucinex DM.
These last a long time and are convenient as they come for over-the-counter remedies.
With either treatment (especially the mucinex), fluid (water is best) intake is especially important to help these work.
To save some money, there are often some generic immediate release pills with guaifenesin and dextromethorphan that are taken every 6-8 hours. I have great luck recommending these if you're dilligent enough to take them. Anything that has the "tussin" aka guaifenesin goes down much better with something citrusy like orange juice to mask the nasty bitter flavor. Despite its horrible taste, though, guaifenesin is very well tolerated and very safe with no known interactions!
If you've already given a dextromethorphan product a try and can't get to a doc, the last resort is to find a pharmacy that lets you sign out a narcotic log for "Robitussin AC" or guaifenesin with codeine. Despite being a class V DEA controlled substance, rob AC is technically over-the-counter in the United States at pharmacies where they sell individual 4 ounce bottles.
Codeine is the best cough suppressant in its class though many complain of sedation and upset stomach.
Finally, at a very last resort, you can combine one of the above products with a Clor-Trimeton or one of its generics. It works mainly as an antihistamine like Benadryl and makes you pretty sleepy like Benadryl too. These sleepy effects are exponentially elevated with other products like ethanol or codeine so be very careful and take it as directed -codeine especially due to its own potent sedative effects. Otherwise, clor-trimeton is great because it is slightly less sedating than benadryl, but has its own cough suppressant properties. So much so that they put it in prescription cough medication such as tussionex.
Start with lots of water, some Mucinex DM (unless you're allergic), and more of that good ol' hot honey and lemon tea. That's all that most folks need to start feeling better without feeling worse first.
Hope she feels better!