I don't know enough about the relationship to actually make any suggestions.....just ask a question.....why can't you suggest very strongly (tell) your Father that you think it would be a much more enjoyable birthday lunch for your Grandma if it was just the 3 of you doing lunch together. Suggest that Grandma has a hard time getting around in a buffet & that a nice quiet small size restaurant that Grandma likes would be a much nicer way for her to spend her birthday lunch. Suggest that some other time all of you could get together, but this birthday would be much nicer spent with just family.

Sometimes we are afraid to say things, but said in the right way, can come across nicely & the right thing can happen with just the right suggestion.....that isn't sounding like excluding the others....just not for this lunch.

Sometimes the suggestion can make the best thing happen when your Father might be afraid to leave out his GF & her Mother....when if you suggest it, then it won't be his idea & then can nicely be postponed to another time.

It might just work?
Debbie