Hi Suzymac,
Keep it very simple, tonic solfa really is easy and will soon be second nature.
The tonic is 'doh', the second 'ray', just like the 'sound of music.'
A good book to compliment the AB Aural test book is 'Aural Time' by David Turnbull. These are practice tests for ABRSM. The one I would recomend for the tonic solfa is Test B. Sight-singing in free time on page 6. David Turnbull recommends singing their letter names or tonic sol-fa names. I use tonic sol- fa as it works every time.
The tests are written in treble and bass so that pupils can choose the ones suitable for their own voice.
I give my pupils a dozen exercises a week to learn on their own at home, out of the book, giving themselves keychords and checking their notes with their own instruments. I show them how to do it at the lesson and then they happily get on with it. It really is easy and they soon become very good at it!
The first exercise is in G major and is doh, doh, doh, ray, doh. It very gradually progresses up to the standard for the AB exam and your pupil will get full marks and suprise themselves!!!