Here are some different ways of achieving your goal:
1. Easiest way: If the signal is strong enough, just let him connect to your SSID when there is a need to share files.
2. You can opt to use publicly available file sharing sites (many are free) that enable you to upload and transfer large files.
3. If the routers support VPN, create a VPN tunnel between the two routers. You will need a static IP or Dynamic DNS service.
4. You can set up FTP and access it via static IP address or a Dynamic DNS Service.
Unless you are technically savvy, # 3 and # 4 are going to be difficult to do.
Miguel Fra / Falcon ITS
Computer and Network Service , Miami, Fl