It is permissible to work in booking hotels inside the Kingdom and abroad for those who want that, unless it is known or thought most likely that they are travelling for sinful purposes, or if the hotel is known for evil and corruption. In that case, it is haraam to tell people about those hotels and it is also haraam to help the one who wants to commit sin to do so, because Allaah says (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment”
But in cases where you do not know, or you do not think it most likely that a person is travelling for sinful purposes, there is nothing wrong with you helping him to book a hotel, and if his aim in doing so is for sinful purposes, then the sin will be on him, not on you.
And Allaah knows best.