Agree with the above - travellers have an exception from the fast should they choose to. The two Muslim pilots I've flown with just added a couple days to the end of Ramadan...but that was during commercial operations. Never had to deal with that instructing where they may have to seek clarification from their Imam whether training would be considered "travelling."
If they feel they must fast I would suggest that, if you do let them fly, make it a morning flight only - that will be closest to their last meal and drink. I would think twice about sending a pilot solo and would only do so on a case-by-case basis.
Ground training seems your best bet, but remember that you'll possibly not have their full attention so don't expect much near the end of the session!
I'd be interested to know what other instructors think about this.