How do you mean that is didnt seem to cover? Is it that the streach marks didnt seam to cover?
Stretch marks are like scar tissue and they start quite red and then fade in to a silvery colour.
If you have sprayed over the area where they are for example ab area then dont worry it wil be fine. No technique is going to make them appear better or worse , it is the clients skin.
In my experience of clients with stretch marks the tan has always made the stretch marks less noticeable. so once the tan develops she should be looking great.
Im my ove experience of stretch marks , rescue oil 2 times a day for 6week plus makes them fade and shrink and are now barely viable.
I wouldnt worrry , youll find in time that you will come across more and more clients with stretch marks. The majority of my clients have them from a few small lines to really big ones.
Just spray as usual thats all you can do, or advise on regular moisturising to minimise them, unfortunately i wouldnt recommend rescue oil while they are getting tans as it will totally ruin the tan and strip it. HTH