Hi.
Since she's a U.S citizen she can keep them in the US. For tax purposes, it makes no difference whether the investments are in Canada or the U.S - the 2 countries have a tax treaty. Does she have family in the United States? If she does, she should consider asking one of them for permission to use their address as her mailing address. Of course they'd have to forward the mail to her in Canada.
Alternatively (if you prefer to keep family out of you business), rent a mail box at a UPS store. Unlike the USPS boxes that come with a P.O Box number, the UPS boxes come with a residential address. At an additional cost, the UPS store can forward your mail to your preferred address. I'd advise you select a store within the bank branch's service State - UPS site has a store locator tool.
Lastly, I strongly advise you have a chat with a wealth manager from Canada, and from the US. The information you obtain from both discussions will help her make a more informed decision.
All the best