Welcome to FT!

If it works - I don't think it is unethical at all: part of the benefit of a Y fare is that you can rebook / cancel / change whenever you want.

However, the cheaper fare likely books into a cheaper fare class than the full fare ticket. This essentially means that, if you purchase the refundable ticket, there is a good chance that the cheaper ticket will no longer be available at the original price.

Also, make you that you meet the minimum purchase requirement of the cheaper ticket. (For example, some tickets require you to buy them 21 days before departure. If she doesn't get her visa 21 days before, the ticket will expire)

Finally - some fares - noticeably those going to the US from Eastern Europe (I believe) have a clause in which non refundable fares become refundable should a visa request be rejected.

Cheers