I think it’s perfectly fine to post whenever. Nobody said that you can’t make Instagram posts or celebrate a holiday while grieving. Sometimes that’s the best time to party because it helps keep your mind off things. If it were me, I wouldn’t want people holding back their fun over my SO’s passing. As long as you’ve showed you’re grieving and you make it clear that you haven’t forgotten about them (and you’re there to support your friend in their time of need… reach out regularly and check on them) I don’t see why an insta post should matter.