Thank you for your reply.
My girlfriend does not have a Belgian bank account yet, so I suppose that no amount would be suspicious in the beginning.
Even when she already has some money on it, it could well be that she had way more on her Serbian account.
We are talking about 5 digits, what concerns me is that my girlfriend could be in trouble because the amount on her Serbian bank account would increase a lot out of nowhere.
Is that something I should worry about?
you need to know the law in Serbia. Let's say you have 10 BTC and you want to send them there for someone to cash without paxing taxes. Are those 10 BTC considered as 'gift' taxable or not in Serbia? Then you want to send this money back to Belgium to HER bank account. Why? You would put her at risk because if one day the tax office decides to look into her account, she would need to justify where the money came from.
As someone else said, your best chances are to cash the money through local bitcoins, buy in online stores without KYC rules etc. I don't know if someone has an off shore account in Switzerland, but that should also be something to check.