Why does he have to stay in Kenya for his R&R? I'm sure that given the circs he will be allowed to return to the UK - mostly for expectant fathers they offer to try and time the R&R for you - but of course it's all guess work, and babies have their own timetable. Some places they don't usually return to the UK, but stay in country or go somewhere closer (am thinking FI mainly) but it's usually theoretically possible for such things.
We managed to time dh's for when dc1 was due from the op tour he was on, but tbh it's usually best to have some sort of a back- up plan in place in terms of birth partner, sibling care, and someone to stay for a week or so after the birth in case of CS etc.
Sometimes it's better to request a switch of tour, and the desk might accommodate (ie he would change for the next guy due to replace him, so would go out 6 mos later) but tbh it's way too late to ask now - it isn't a surprise that his wife is due in march! He's known for some time, lol. And it depends on his trade, and how harmony looks, how hard it is to fill posts for his specialty etc.
She will be better off making her own plans that don't include him, and keeping her fingers crossed. Tbh I'm surprised she's just asking now. This should have been discussed yonks ago when he received his detachment details.
She can rest assured that if anything untoward happens, they will get him back as fast as poss though - she needs to make sure the contact details are in a prominent place in the house so that she or someone else can notify the mod if necessary. Not to wish doom and gloom, but important for her and her nok in country to be prepared, particularly if she is not on a patch etc.