This reminds me of a time when we invited a mutual (female) friend over for Sunday lunch. We know her through a sport that we all play and she lives about an hour from us and is always complaining about being lonely and not having friends, etc. Anyway, it took us ages to find a date that we could all do and after she'd agreed on a Sunday with us, she emailed to say she was going to have a friend staying with her that weekend so would it be OK if she brought her along too!!! (someone that we had never me, to have lunch at our house - and this from someone who has no friends, is lonely, etc!). I feel really peeved! In the end, I emailed her back to say 'If you have a friend staying, why not enjoy the weekend with her and we can rearrange our Sunday lunch with you'. In the end, she came anyway but minus the friend - because her friend ducked out (probably realising it would be rather rude!!). So people can be very thick-skinned! I would do as others have advised and just say something along the lines of 'I thought it would be nice, just the two of us - let's do it another time'.