Where you live will make a difference - if you're in East Reading then the Waterloo line from Earley will be best. Assuming central Reading, the obvious choices first:
Reading to London Paddington takes approx. 30 minutes, and the Tube another 20 or so. This is the most expensive, but fastest option. The trains are also very busy and she is likely to have to stand frequently. Cost:
£477 per month (inclusive of unlimited travel within London).
Reading to London Waterloo takes approx. 90 minutes. Waterloo is about 20 minutes walk from Leicester Sq (and Charing Cross which can be reached with a change at Waterloo on the same ticket is even closer). These trains start at Reading so she's guaranteed a seat on the way in. I am sure there was a cheaper season ticket for this route but it doesn't appear now. Cost:
£392.50 per month (valid to Paddington as well, but not on the Tube).
Reading to London Paddington on the stopping trains takes approx. 40-60 minutes depending on the service, plus Tube. Using a combination of tickets this can be done for
£391.40 per month including unlimited travel within London. This is using a Reading-Maidenhead season and a Maidenhead-London Travelcard. The trains she uses must call at Maidenhead - there are a few trains at peak times which run non-stop between Maidenhead and London so don't take much longer than the Reading-London fast trains. Note that this combination may cause trouble if she wants to use the Waterloo-Reading route.
These are the options I would recommend and which will not cause problems. It is possible to make the journey considerably cheaper, however I would
not recommend these tickets to someone who wants a hassle-free journey.
Whatever ticket you choose, remember that season tickets can be bought for any period from 1 month to a year - so if you're going on holiday in 2 months time, ask for a ticket valid up to the last day you need it. That way you can avoid paying for unnecessary validity.
Finally (and without wishing to sound cheeky), it's not clear if your wife already has a place at Kaplan or is just planning to go there. If money is an issue, there are several good English schools in Oxford which is a much cheaper and more pleasant commute.
I imagine there are also regular coach services from Reading to London. Again these can be slow as they are caught in traffic congestion.
You'd think so wouldn't you. However, there aren't.