People who vomit a lot can get dehydrated and have reactions resulting in low blood pressure, making it difficult (and sometimes dangerous) to stand up in a lav. They may need to lie down or sit to avoid passing out, if it's bad enough. The pax is the best judge of if he feels up to walking to the lav.

The FA should try and make sure the vomiting passenger is safe and not so sick that the flight will have to make an emergency landing, or so sick you have to call for a doctor overhead.

I don't know what NW policy is, but on a recent WBC flight I was on DTW-FRA a pax started violently vomiting (was it the fish dish or all the refills on wine?) and the helpful FA, after making sure the pax had extra emesis bags, immediately moved the seatmate to another seat (he immediately accepted the offer )and then helped assist the vomiting pax with comfort and clean-up. She did not or could not rush him to the lav (he went there shortly after things settled down, embarassed, to change his clothing).