I have epilepsy and also have a very healthy absolutely fine two year old. Unfortunately some people don't get the right advice or aren't able to take the drugs that carry less risk of causing birth defects. But many women with epilepsy are left on sodium valproate or Epilim because nobody ever bothers to think 'Oh, this patient is of childbearing age, better review her medication'. Which is outrageous.
There are anti-epileptic medicines which carry a far smaller risk of birth defects - I take lamotrigine (Lamictal) which is one of the 'safest' as far as anyone can tell. She also needs to start taking high-dose folic acid as soon as possible as the medicines inhibit folic acid absorption PLUS one of the potential birth defects is spina bifida, which is why everyone takes folic acid in the first place. I was on 5mg daily.
She should have an appointment with a neurologist who can talk her through the options on medication and the risks.
Generally the risk for women with epilepsy is said to be double that of the rest of the population - but it's more complicated because people take different medicines in different combinations at different doses and have different symptoms. But even with a doubled risk, that means the vast majority of babies born to women with epilepsy are absolutely fine. Personally I wouldn't get pregnant if I was taking sodium valproate/Epilim though, but that's just my personal take on the matter.
What she has to decide is the trade-off between the risks of taking the medicines and the risk of seizures. Most doctors would say you are better off taking the medicine and controlling the seizures. I'd agree but try to get switched onto a medicine that carries the lowest risk - and avoid TTC until I was stabilised on that medicine.
The dose may need to be upped in the third trimester because you have an increased volume of blood swilling round your system, so the dose of any drug is less concentrated. I waited to see what would happen and had a couple of seizures as a result. But the likelihood that the drugs will cause defects in the third trimester is low because all the organs have been formed, baby is just growing (IIRC). If she does get pregnant, she'll probably be offered an extra heart scan in late pregnancy too.
HTH and hope everything works out for your friend.