Hello
I’m not a doctor but Michael Mosley and Michelle Harvie (of the other 2:5 diet) are and they have pioneered these diets.
From reading both books and their rational: I would say that it’s quite the opposite-the whole point of the fasts are to rest your body from digesting food so it can go into repair mode.
They always say though: if in any doubt consult your GP.

Hello, threelionsfan – I think the term used, in a medical sense, is that fasting ‘stresses’ the body by the sudden reduction of calories – usually just for one day, in the case of the 5:2 approach – and that stress provokes various positive changes in the body’s hormonal responses, that lead to many suggested health benefits. There is no negative connotation to the word ‘stress’ in this context and I agree with PreciousBooBoo’s comments about the body’s resulting rest and repair.
If you have time, read around the different threads on the forum and maybe that will give you an insight into the improvements experienced by many people using the 5:2 Intermittent Fasting system, in energy levels, feelings of well-being and measurable health markers (over and above the straight-forward advantages gained by losing excess weight).
For some reason, many people like to stay sceptical and gain-say successful health regimes, without necessarily educating themselves on the subject or trying the method for themselves. Perhaps you might suggest to your colleague that s/he needs to read the book, to learn the actual facts, and then, hopefully, there will be no need to doubt? All best wishes.