I'd say no, they are not dating, it's just a one night stand.
for now anyway.
I'd say the spirit of the rule was to discourage him from acting like a horndog around five defenceless women, and also to prevent making things awkward within their group. The former is arguably not the case here, but the latter is totally gonna happen
I'd also argue that this is not a one night stand, in those, both parties have full acknowledgement of (lack of) consequences and of the fact that they'll be on their merry way once the night ends, no hard feelings and all. This, on the other hand, is gonna make things awkward at the very least.