Count Bobbert of Frugundy has many children, and he has sent his third son, Preter, to be raised at the court of Duke Wudlig of Brutsels, both to honour his ally and to allow the child to benefit from the many scholars and clergymen in the court of Brutsels and receive an excellent education. But, Wudlig is devious and he knows that the count's first two sons are sickly. He is counting on this child to inherit the county, and become his neighbouring ruler. So, Wudlig subtly sabotages Preter's education. He arranges for noisy military parades to take place right outside Preter's study room, subtly encourage immoral behaviour and sins, and makes excuses why no wooden swords are available for practise today. Ten years later, Preter returns to his father's court unable to tell a motte from a bailey. Bobbert is furious but there's no way to tell that it wasn't just Preter's mental inadequacy all along. Relations between the fiefs sour, but eventually Preter does inherit and the poorly-ruled county is duly conquered by Wudlig or his heir.
The above paragraph is an illustration - don't take those characters literally or this question would be story-based!
Foreign education for nobility was definitely a thing - I see it referenced in a few places but cannot find a source specifically about it. My question is: would a lord sending their child away have reason to fear that they would be raised inadequately due to malice on the receiving lord's part? This doesn't even have to be so overt as sabotage, it could be as little as them doing less than their absolute best to raise their future neighbouring ruler. So an alternative phrasing is: could a lord always count on their neighbours to raise their foreign children the best they can?
A third possible phrasing (you can tell I'm having trouble wording this question xD): could a feudal lord under certain circumstances, either overtly or subtly, sabotage the education of a foreign noble heir in their custody, without suffering consequences worse than what they gain by having a weak neighbouring lord? And if so would those circumstances be commonplace enough that the average lord sending their child would have to consider malice? If this is technically possible but only under extreme circumstances (like the recipient living on a remote island) then for general purposes the answer to the question would be no.
Assume generic feudal rulers who are not unusually sadistic or unconcerned with self-preservation - just regular people exercising realpolitik. Do not invoke plot-based or otherwise person-specific relations like the two rulers being former lovers; make your answer apply to any two lords that might engage in the medieval student exchange program.
Finally, don't assume things about the setting besides the place being divided into feudal realms, and anything unmentioned that is crucial for that setting to make sense (e.g. a class structure, pre-industrial technology). I am not asking this on History.SE so "he can't because the Pope would be angry" is not a valid answer here.