I think it's a great idea, assuming that Diana is the sort of person who likes parties and would have a great time being made a fuss of. I wouldn't read it at all like 'aww, poor you, you're single'. Maybe a little bit as a substitute for a wedding, but not for a partner, if that makes sense. But, I don't really have a problem with that. It's been commented on before that it can sometimes be harsh to be an unmarried person who goes to wedding after wedding but doesn't have a big 'me day' themself, especially if they would love one.
It sounds like Diana has a big heart, if she's going out of her way to do awesome things for Emily, so hopefully she would take it in spirit of 'we want to make a fuss over you, too, like you've done for us!' and not 'aww, poor single girl will never have a wedding'.
In my group birthday celebrations aren't very 'equal', a lot depends on timing and who wants what--some of my friends have had big gifts done by collection by the group, others have had huge parties, some don't even tell us when their birthday is...I don't know, if one of my friends had a massive birthday party just cause, I wouldn't think it was weird or question it at all. Just figure 'oh, great, we haven't had a party in awhile, great excuse!'