what are the laws on putting mascara on on shabbat?
Better to use the waterproof and put before Shabbat.
What if she goes to the Mikva on Friday night?
The waterproof will help, if you can find a Mikva that will let you go in with Mascara. OR as an alternative you can go to Mikva before Shkia/Sunset on Friday. Some communities allow that. However once you get home, your husband would have to leave to avoid you both being home together. He can come home after 30 minutes after Shkia/Sunset