If he operates two uniquely different enterprises there is no reason he cannot continue part of his business as a sole proprietor. The issue, though, is if there is any overlap he may "pierce the veil" of liability protection offered by operating within the LLC. Better to switch completely, although he may have to file Schedule C as sole prop for the first half of year and 1065 for second half.
Yes, he should "sell" his sole prop assets to his LLC, instead of just using them. Back to the liability protection and insurance coverage purchased by LLC. He sells it by making a Member Contribution (equity increase).
