~に花(はな)を持(も)たせる (hana o motaseru) literally means ‘to let ~ have flowers’. Figuratively it indicates the act of generously letting someone look great and commendable.
It is often used when you let someone take the credit for something that you have contributed to a lot. Another example is when you deliberately underperform someone and let them win a game/match/competition.
「これも全部(ぜんぶ) 田中先輩(せんぱい)のおかげです」と言(い)って、先輩(せんぱい) に花を持たせた
I said, “This is all thanks to Tanaka-Senpai”, to let him “have flowers” (take the credit for success)
〜を立(た)てる: literally “make ~ stand” and figuratively “flatter/please ~”, “act obsequiously towards ~”
忖度(そんたく)する: “please someone by considering their wishes”
遠慮(えんりょ)する: “to hold back or refrain from doing something out of consideration for others”
空気(くうき)を読(よ)む: literally “read the air”, and figuratively “read between the lines”, “read the room”
忖度しない: without 忖度
遠慮しない: without 遠慮
空気を読まない: without 空気を読む
Related Post: Funny Japanese Idioms about Plants