Buy cell phone in US?

I have a dear friend going to the US for up to 4 months. She has a very old phone and would like to get a new one. Would u say its best to buy a phone in canada, or if u buy a US phone can it be used in Canada? Are there disadvantages to buying a phone in the US?
Appreciate any feedback.

Buying a phone in the US outright would work fine IF she has US money to buy it with. Buying it using a Canadian account will result in a pretty awful conversion factor -- so a $1K iPhone becomes a $1.45K iPhone instantly!

Also, she would need to be careful that the iPhone she's buying in the US isn't locked to a carrier. Canada got rid of that restriction some years ago, but not everywhere in the US did.

An iPhone bought in the US will work fine up here once she gets a Canadian SIM card from her carrier.

I'd recommend she get the iPhone here, and sign up for a plan that offers US/Canada calling/text/data. Freedom Mobile offers such a plan, but so do some of the budget carriers (though theirs may only be 4G, not that it makes a huge difference).