I’m surprised that the effect of major rivers is big enough to be visible on a global map, at least in otherwise saline areas (Amazon, Mississippi, Congo?). Interestingly, the world’s longest river (Nile) which drains into one of the saltiest seas (Mediterranean) doesn’t register on this scale at all.
I have a similar background and went to Flanders for a Master’s, and have been very satisfied.
There’s several good universities, and most everybody speaks good English (even outside of the university environment) - to the degree that it can even be difficult to practice Dutch unless asking people deliberately to do so. Nevertheless, learning some Dutch helps with some everyday things like the supermarket, and should you aim for an intermediate+ level (think B1/2) it can give you an edge when looking for a job later.
Job opportunities are generally better than in most European countries, both in the industry and with all the organizations hosted in Brussels.
University fees are low compared to the UK (though similar to other European countries), but living costs are high. Expect to pay at least €400 rent for a small student room, and monthly expenses of another €400 if living in simple student conditions.
Public services are mostly good, with decent education, excellent healthcare and mostly reliable public transport (trains are alright, but buses can have issues from time to time). Most places are safe (besides the occasional bike theft), and police are actually nice.
I’ve grown to like the people here. Coming from southern Europe they can appear somewhat closed, and indeed tend to keep to themselves mostly. But if you show genuine interest in the country and it’s ways, and even start learning some Dutch, it will be appreciated and you’ll find people to hang out with and even become friends (of course, the alternative to just mingle with internationals is very common). In general they’re polite, helpful and professional even when they don’t know you, it’s just that it takes a while to break this first “acquaintance” barrier. Sharing a dorm with them is usually a good way to get to know them better.
Quality of life depends on who you ask. The weather is pretty much what you’d get in England, and night life is limited to a few places and not too long either. But there’s a lot of opportunities for activities, sports, culture etc., and the region has a rich heritage with beautiful cities (also some nice nature in Wallonia). Work-Life balance is excellent which coming from the South is a very welcome change (Belgians will work well and efficiently until their shift is done, and then not touch work again until the next day).
If you have any specific questions, I’m happy to answer them :)
Most general “must-see” places have already been mentioned, you can’t go wrong with them. I would perhaps throw in Leuven as a good half/one day trip, especially if you’re staying in the center of the country.
I’d suggest to also focus on distinctly Belgian food/activities, which your friend might not have in Germany: visiting a frituur, sampling the local beer/pralines, going for a bike ride, perhaps a cantus if you’re up for that. These are independent of location but are all quintessentially Belgian.