I've never considered this to be a moral dilemma, and I still don't. All sites are first-come, first served. The other group(s) should also know that and be prepared for contingencies. If it was storming and I saw a group looking for a site, I would invite them in, and share the tarp & coffee, & a meal if we were preparing one anyway. Then, when the weather clears, I would bid them farewell, as they find their own site.