Answer
(Fatwa: 1730/D=244/K=1430)
In Islam, a Muslim man was permitted to marry up to four women at a time. If your friend fears to be involved in adultery and he does not get sexual satisfaction from his first wife, he may be allowed to have marriage with the condition mentioned in the Quran itself i.e. he is able to fulfil their rights with equality and justice, otherwise he should keep fast (to break the extra sexual feelings) or keep himself content with the first wife.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband