Why do you think that Jane Rochford would assist Catherine Howard in committing adultery? She knew Henry’s wrath and that she might be implicated.
Tough one! We just don't know what Jane's motives were for acting as a go between for Catherine and Culpeper. Some historians (e.g. Lacey Baldwin Smith) have seen Jane as an agent provocateur and perhaps she enjoyed the role of go-between, living her life through Catherine and Culpeper, but it could be that she was simply following Catherine's orders. Jane was, after all, simply a lady in waiting there to do the bidding of her royal mistress.