Try putting a rag soaked in her wee in the garden and some of her poo (yuk but bear with me).

When it is wee/poo time take her out on a lead so she doesn't get distracted playing. Just stand still and wait until she goes. Up the treats to something really special and just use that when she goes in the garden (i know that may not be easy because of her tummy trouble but try and find something she really goes mad for)

Perseverence and patience will pay off in the end.