He wants to spend too much on his sister.

So my bf’s sister birthday is coming up. She asked him for brand makeup & VS lotions/perfumes. Now, I have no problem with him buying things for anyone. It’s just that he has other payments he has to take care of & he wants to buy her an iPhone 8 along with all that she said. I’m gifting her some shoes that I never wore because they didn’t fit me. I also hooked him up with a friend who sells brand makeup cheap. We’re going to the mall this weekend to get her perfumes. It was all good until he started mentioning the iPhone 8 and all of that because like I said, he has bills to pay here so it’s kind of unreasonable to spend so much & then have to ask to borrow money to pay rent or something. He’s already done it once months ago when he spent a lot of money going out with his friends. Should I tell him something (what?) or just let him spend HIS money?