Either change the key, or don't do the song. End of story.
Singing hours at a time, you absolutely need to look after your voice, if you're straining then you're going to get nodules eventually. Technically, if your voice feels sore after singing, you're doing it wrong.
It's likely that the guitarist just can't be bothered changing keys. Is that worth ruining your voice? Put your foot down.
I've been playing in cover bands for years, just on backing vocals and bass. our singer was completely professional but would need a few keys changed now and then and we would also take into account where the difficult songs were placed in the set.
Start with a few easier songs on your voice to make sure you're warmed up. Then get the tougher ones out the way while you've got the pipes for it, end on relatively comfortable songs as you'll likely be tired.
Edit: Oh, and stay hydrated. Massively important.