My little one was around a year old the first time I was on my own in the house with him overnight, however my partner spent some of the early months sleeping in a separate room so I was essentially alone overnight then. However it was easier psychologically because I knew he was there if I needed him, and I could call him if there was a nappy leak or if I just needed a break. I'd be inclined to say that your partner should say to his friend that he'd very much like to come but he might have to pull out at the last minute, as you just don't know how you'll be finding things. Obviously it depends whether he's ok with losing out financially if he can't go.
My partner also has a stag do about 3 weeks after my due date. It’s only one night away and we live right near some really good friends and my mum who can all come over and help but I feel the same and I’m a bit anxious about it
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Hey my partner sometimes works away and our LG was 4 weeks old when we had our first nights just me and her as he had to work away for 3 days/4 nights. I found it hard but just tried to have things ready to make things easier for myself. Also don't stress about trying to keep on top of things while he's away just focus on you and baby as other things can wait. You'll be okay! đź’•