I don’t know anything about vape pen technology, but I can tell you that putting water in there probably is a very bad idea. I think they are only designed to operate on a specific type of fluid/oil. DO NOT PUT WATER, OR ANYTHING THE PEN IS NOT DESIGNED TO USE. If you do, you risk damaging or destroying the pen, which your mom would be much more angry about.
It was a very bad idea to touch your mother’s vape pen to begin with, as:
1) THC is harmful to the brain of a growing person (I’m assuming you’re a teen)
2) Vaping is unhealthy for the lungs
3) It’s not yours
Is a short-lasting high worth all the potential UNCURABLE negative health effects in the future, or your mom getting angry at you? Just consider that next time before you reach for that vape pen.
So now you know not to do it again. What now?
Well, if you know the exact type of oil your mom uses and somehow obtain it, you could use that to fill it up.
Otherwise, you have no options other than to hope your mom doesn’t notice.