A questioner at the language forum 'Pain in the English' asked, which is correct?
- Assists attorney in drafting documentation.
- Assists attorney with drafting documentation.
The few people that commented seemed to agree that the first was correct, and there was one suggestion that 'assist in' is followed by a verb, whereas 'assist with' is followed by a noun.
Both in and with are prepositions, so the only verb form that can follow either of them is a gerund, which is in fact a verbal noun, and there doesn't seem to be any grammatical reason that I can think of why a gerund can't follow 'assist with', nor any reason why a standard noun can't follow 'assist in'. But perhaps there's an idiomatic one.