is it soreness or bruising? there is a difference.
1. If it's bruising (ie if it hurts when you press it) it could be her saddle is too small so she's not sitting on her sit bones, but the soft tissue in between. Get the bike shop to measure her pelvic sit bones and get a saddle wide enough for them (this was my problem and i put up with it for years without realising... can't tell you what a difference an inch makes (oo-er)!
2. If it's soreness (chaffing) then this could be caused by (a) too big a saddle or (b) not soft enough pad in her shorts, or (c) simply just not being used to being in a saddle. If (a) see point 1 above, same principal applies, if (b) get some nice shorts or if (c) she will get used to it, but to relieve it in the meantime Aqueous or Sudocrem will be fine. I use sudocrem and never got thrush from it, but i'm not really prone to thrush anyway. I actually find on my Paris trip, that using a cream while riding just makes the rubbing worse, so i used the cream after each day had finished. (i don't need it now, like i said, she'll get used to it).