What is a product most frequently bought together with
Two approaches, both based on discrete mathematics. The first one is a set-theoretic approach to frequent item-set mining and association rule mining. For instance A-priori algorithm works here.

the next approach is creating a graph based on orders and do graph mining on them. A-priori might be a better approach as in this case you probably need to deal with hypergraphs which makes life a bit complicated.

I suggest you first extract sets of different orders from your dataframe. As your problem is not general and you already know that there is only association to one product is important, you can prune your data much. Then simply rank other products bought together with Bike Helmet by counting number of their occurences and dividing by number of all (it gives you the probability estimate of each product is bought together with bike helmet).