English term (edited): in lieu of sympathy flowers

You are parsing it incorrectly , it is 'sympathy flowers' (and 'congratulatory flowers')

Usually, 'sympathy flowers' would be = 'floral tributes' when someone dies; however, there is just a possibility that here it might mean flowers out of sympathy when someone is ill etc. — but I somehow doubt that the word 'sympathy' would be used for that, as it has so much of a connotation of funerals etc.