Travel between Canada and USA

My mom is planning to fly from Canada to the USA to stay with me for like 3 months, We currently have booked only a 1 way fare (because of a sale), will she need to have a return fare booked before her travel or can we book it after she arrives here in the US? She is a Canadian citizen. We were considering driving her back to Canada, so there really wouldn't be any proof of her return date. Any help is great appreciated!

she will need her passport to fly into the US of course.

She will be asked about how long she is going to stay etc.. but if she is clear she is only on a visit, staying with relatives (hence needs no support) and plans to leave within a 3-4 month time frame and that her children are driving her back (to explain no return ticket) she shouldnt have a problem.

No, Canadian citizens are not required to have round trip tickets when they visit the U.S.

She should be fine. They might want to know who she plans to stay with so it would help if she has your address and phone number handy when she goes through customs.