I have done a bit of looking on this in the past as was going to try and make some for the kids teams. I think it is going to depend on if she has access to photoshop or not and how many looking to do. If she is looking to sell commercially while using a template, I might try and look at these sites as a suggestion:
https://graphicriver.net
https://creativemarket.com
https://www.privateprize.com
https://photosolutionsmarket.com
The good news on the above is you purchase the template of your choice for commercial use and then can use it unlimited (unless the terms of agreement are different) and not have to pay anything further. However, most of them are only a single template, so if looking to build a portfolio of different templates, may need to purchase a number of them upfront. So, far I have only used GraphicRiver. I have been pleased with the downloads I have purchased there for some projects. I haven't done any trading card ones, but... The last two offer template downloads for professional photographers which look really nice. I am pretty sure we will be getting a template from photosolutions for the kids upcoming season to try out, but haven't yet so nothing to report.
Other options are the create your trading cards online and order from a service. These you can just upload the photos you want to use and organize online to their templates and they print them and send off to you. Nice thing on these websites is no upfront costs and have a wide varierty to pick from, however, they are usually costly (especially if only ordering a few).
I hope that helps and good luck with the search.