The good news is, your client can take advantage of the imagery even if she drops into a sound sleep every time she listens. In fact, many of my colleagues would argue that the imagery will go in deeper and get absorbed better if she's sleeping - those lovely, slow, super-receptive theta and delta brain wave frequencies, engendered by a nice snooze, are just the ticket. So don't worry about that and don't try to change it.
If, however, your client is interested in knowing (consciously) what's on the audio, she can listen while taking a stroll or standing up, leaning against a wall.
But it's not necessary.
It turns out there are a lot of people who just can’t be relaxed and awake at the same time. So they fall asleep the second they relax. In your client’s case, this could also be conditioning from childhood. Lucky her! A lot of people have trouble falling asleep!