Hi Rosie
Not sure about this one as I don't use any essentail oils on pregnant women; when they come for a massage I just use a carrier oil. I think I would reccomend that she ask her doctor for something to help clear it up.
I didn't find Tea Tree on its own very helpful when I was trying to clear up athletes foot. I made up a mixture of three anti-fungal oils, put them with some carrier oil then put some in a bowl of water to soak my feet in. It worked like magic, the athletes foot was gone within 3 days. The oils I used were manuka, rosemary and german chamomile (matricaria recutica)
Love and blessings