Really depends on your budget.

If you are going for something below 20 usd the maybe a basic thing like: this https://www.amazon.com/dp/B08HY45RL7/ref=vp_m_pbcmv3_TIER1_cml_lp_B07VHTG9PB_pd?_encoding=UTF8&pf_rd_p=7745cb35-13f2-434a-bcb8-d080358f430c&pf_rd_r=0GS45TTCHMJ6CTQWY11A&pd_rd_wg=ZjpbI&pd_rd_i=B08HY45RL7&pd_rd_w=dcnvE&content-id=amzn1.sym.7745cb35-13f2-434a-bcb8-d080358f430c&pd_rd_r=d10fa971-9bb9-4dbe-aa35-50a535ed9d99

If you want to get something more professional then for 50 use you can get something like this https://www.amazon.com/24-Piece-Bartender-Measuring-Corkscrew-Professional/dp/B08XPYNGRB/ref=mp_s_a_1_1?crid=2JP0J4CQG44IR&keywords=boston+shaker+set&qid=1671197227&sprefix=boston+shaker+se%2Caps%2C552&sr=8-1

Point is basic sets aren't too expensive but you can get a decent set for 50 usd, I own both of these kinds of sets and they work great. Would recommend boson shaker($50) but the other one works just fine too. Just look up 'cocktail shaker sets' and see what one you wanna get your friend.