All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We advise the sister not to help her son to go and study in non-Muslim countries, and do her best to prevent him from going since he is studying in Muslim countries and he is doing well as mentioned in the question.
There are many harms and disadvantages in travelling to non-Muslim countries mainly for a young man at the bloom of his age. Indeed, a wise person is the one who learns the lesson from others. How many young men who went to study in non-Muslim countries and then lost their religion and moral standards, and became means of destruction to their own nation? The sister should know that she is going to be held accountable for her son on the Day of Judgement. The Prophet said: "All of you are guardians and will be questioned about those under your guardianship…,' [Al-Bukhari and Muslim].
Therefore, she should endeavour to convince her child not to travel to non-Muslim countries due to the harm and disadvantages of residing there. She could mention to him the evidence which warns against residing in non-Muslim countries like the saying of the Prophet : "Whoever keeps company with a non-Muslim and resides with him, is like him." [Abu Daawood] The Prophet further said: "I am free from any Muslim who resides among non-Muslims." [Abu Daawood]
It is only a matter of few years, and he will finish his studies, as he has already completed five years. For more benefit, please refer to Fataawa: 81642 and 88291 in relation to residing in a non-Muslim country and Fataawa: 84777 and 82091 in relation to studying in a non-Muslim country. Allaah knows best.