We chatted to the ladies at the Body Shop and this is what they had to say:
Because it was never trialled on Eye lash extensions it is hard to say for sure!
There is a small possibility that it could affect the glue that fixes the lash but in terms of safety it would be fine.
Brow and lash gel is effectively a hair gel for lashes so in theory it would just form a light film over them.
Hope that helps!