Is it permissible to partner with a government employee who has suddenly become wealthy, knowing that there is suspicion surrounding the source of his money, and is it obligatory to inquire about the source of his money before partnering with him?

The default is to assume the best of a Muslim, to find excuses for them, and to avoid much suspicion and sin, and spying on Muslims, as Allah the Almighty said: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy." There is no harm in dealing with someone whose acquisition of unlawful money is suspected, and one is not required to inquire about the source of their wealth. However, if it is certain that their money is unlawful, or if it is predominantly believed to be so based on clear evidence, then dealing with them is impermissible. It is advisable to avoid dealing with them when in doubt, as the Prophet (peace be upon him) said: "Leave that which makes you doubt for that which does not make you doubt."