Not quite as simple. You're forgetting the inflation rate differential (our exchange rate should depreciate at an annual rate of our inflation minus UK inflation over the long term). The interest Frankie's saving will not be worth much in pounds sterling in a few years. In addition, if he invests in global equity, he'll have that growth on top of the inflation differential.
I would love to argue this assertion but I am too tired (after a long day at work) and lazy to do the research (to make sure of the facts).
...but even without the research to back it up, the exchange rate is affected by a lot more than the two respective inflations. Further to this the worth of Frankie's interest saving does not need to be worked out in a few years via your exchange rate calculations as it will be an immediate saving right now... he will no longer be paying interest right now.
You also appear to have made another assumption and that is that his property value will not increase over the period before he leaves... or at worst you have just failed to factor it into your calculations...
You are also basing your entire investment decision on rand hedging. This is fine and dandy but it is based purely on the premise that the rand WILL depreciate against these currencies. I know a number of people who, based on the general advice at the time, did that exact same thing in the late 90's and early 2000's, and a lot of them are only now making that money back. It is not as sure a thing as you appear to be asserting. Yes, it may happen, as it may have back then... but it also may not! His interest on his bond, however, is GUARANTEED!
Anyway I said I would not argue it so let me leave it at that...
EDIT : OK did a little bit of research :
Between January 2003 and January 2005 the exchange rate went from R14.58 per pound at one point down to R10.80 per pound at one point .
Over the same time the UK had inflation annual inflation of 2.1 and 1.4% for 2003 and 2004 respectively.
South Africa had inflation of 9.9 and 5.9% for 2003 and 2004 respectively..
This is just an example but who says this could not happen again over the next two years....