What is the ruling on dealing with lawful money with a person whose money is mixed with unlawful elements?

If by "dealing" participation is meant, then do not participate with someone whose wealth you suspect was acquired through unlawful means, due to the Prophet's (peace be upon him) saying: "Leave that which makes you doubt for that which does not make you doubt." And you should advise that person to repent to Allah and to seek lawful earnings in his livelihood.